f2fs: fix to cover sentry_lock for block allocation

In the following call stack, f2fs changes the bitmap for dirty segments and # of
dirty sentries without grabbing sit_i->sentry_lock.
This can result in mismatch on bitmap and # of dirty sentries, since if there
are some direct_io operations.

In allocate_data_block,
 - __allocate_new_segments
  - mutex_lock(&curseg->curseg_mutex);
  - s_ops->allocate_segment
   - new_curseg/change_curseg
    - reset_curseg
     - __set_sit_entry_type
      - __mark_sit_entry_dirty
       - set_bit(dirty_sentries_bitmap)
       - dirty_sentries++;
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1167,6 +1167,7 @@ void allocate_data_block(struct f2fs_sb_info *sbi, struct page *page,
 	curseg = CURSEG_I(sbi, type);
 
 	mutex_lock(&curseg->curseg_mutex);
+	mutex_lock(&sit_i->sentry_lock);
 
 	/* direct_io'ed data is aligned to the segment for better performance */
 	if (direct_io && curseg->next_blkoff)
@@ -1181,7 +1182,6 @@ void allocate_data_block(struct f2fs_sb_info *sbi, struct page *page,
 	 */
 	__add_sum_entry(sbi, type, sum);
 
-	mutex_lock(&sit_i->sentry_lock);
 	__refresh_next_blkoff(sbi, curseg);
 
 	stat_inc_block_count(sbi, curseg);