vfs: add explanation of I_DIRTY_DATASYNC bit

Add explanation of I_DIRTY_DATASYNC bit.
Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Joern Engel <joern@logfs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/fs.h

@@ -1284,8 +1284,10 @@ struct super_operations {
  *
  * I_DIRTY_SYNC		Inode is dirty, but doesn't have to be written on
  *			fdatasync().  i_atime is the usual cause.
- * I_DIRTY_DATASYNC	Inode is dirty and must be written on fdatasync(), f.e.
- *			because i_size changed.
+ * I_DIRTY_DATASYNC	Data-related inode changes pending. We keep track of
+ *			these changes separately from I_DIRTY_SYNC so that we
+ *			don't have to write inode on fdatasync() when only
+ *			mtime has changed in it.
  * I_DIRTY_PAGES	Inode has dirty pages.  Inode itself may be clean.
  * I_NEW		get_new_inode() sets i_state to I_LOCK|I_NEW.  Both
  *			are cleared by unlock_new_inode(), called from iget().