f2fs: fix to data block override node segment by mistake

v4: Rearrange the previous three versions.

The following scenario could lead to data block override by mistake.

TASK A            |  TASK kworker                                            |     TASK B                                            |       TASK C
                  |                                                          |                                                       |
open              |                                                          |                                                       |
write             |                                                          |                                                       |
close             |                                                          |                                                       |
                  |  f2fs_write_data_pages                                   |                                                       |
                  |    f2fs_write_cache_pages                                |                                                       |
                  |      f2fs_outplace_write_data                            |                                                       |
                  |        f2fs_allocate_data_block (get block in seg S,     |                                                       |
                  |                                  S is full, and only     |                                                       |
                  |                                  have this valid data    |                                                       |
                  |                                  block)                  |                                                       |
                  |          allocate_segment                                |                                                       |
                  |          locate_dirty_segment (mark S as PRE)            |                                                       |
                  |        f2fs_submit_page_write (submit but is not         |                                                       |
                  |                                written on dev)           |                                                       |
unlink            |                                                          |                                                       |
 iput_final       |                                                          |                                                       |
  f2fs_drop_inode |                                                          |                                                       |
    f2fs_truncate |                                                          |                                                       |
 (not evict)      |                                                          |                                                       |
                  |                                                          | write_checkpoint                                      |
                  |                                                          |  flush merged bio but not wait file data writeback    |
                  |                                                          |  set_prefree_as_free (mark S as FREE)                 |
                  |                                                          |                                                       | update NODE/DATA
                  |                                                          |                                                       | allocate_segment (select S)
                  |     writeback done                                       |                                                       |

So we need to guarantee io complete before truncate inode in f2fs_drop_inode.
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NZheng Liang <zhengliang6@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/super.c

@@ -915,6 +915,10 @@ static int f2fs_drop_inode(struct inode *inode)
 			sb_start_intwrite(inode->i_sb);
 			f2fs_i_size_write(inode, 0);
 
+			f2fs_submit_merged_write_cond(F2FS_I_SB(inode),
+					inode, NULL, 0, DATA);
+			truncate_inode_pages_final(inode->i_mapping);
+
 			if (F2FS_HAS_BLOCKS(inode))
 				f2fs_truncate(inode);