ext4: fix an incorrect comment about i_mutex

i_mutex is not held when ->sync_file is called.
Reviewed-by: NJan Kara <jack@suse.cz>
Signed-off-by: NAndy Lutomirski <luto@amacapital.net>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/fsync.c

@@ -109,8 +109,6 @@ static int __sync_inode(struct inode *inode, int datasync)
  *
  * What we do is just kick off a commit and wait on it.  This will snapshot the
  * inode to disk.
- *
- * i_mutex lock is held when entering and exiting this function
  */
 
 int ext4_sync_file(struct file *file, loff_t start, loff_t end, int datasync)