ext4: honor O_(D)SYNC semantic in ext4_fallocate()

Ext4 must make sure the transaction to be commited to the disk when
user opens a file with O_(D)SYNC flag and do a fallocate(2) call.

This problem had been reported by Christoph Hellwig in this thread:
http://www.spinics.net/lists/linux-btrfs/msg13621.htmlReported-by: NChristoph Hellwig <hch@infradead.org>
Signed-off-by: NZheng Liu <wenqing.lz@taobao.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/extents.c

@@ -4420,6 +4420,8 @@ long ext4_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 		ext4_falloc_update_inode(inode, mode, new_size,
 					 (map.m_flags & EXT4_MAP_NEW));
 		ext4_mark_inode_dirty(handle, inode);
+		if ((file->f_flags & O_SYNC) && ret >= max_blocks)
+			ext4_handle_sync(handle);
 		ret2 = ext4_journal_stop(handle);
 		if (ret2)
 			break;