[PATCH-v4 1/7] vfs: split update_time() into update_time() and write_time()

[Christoph Hellwig]

As mentioned last round please move the addition of the is_readonly
operation to the first thing in the series, so that the ordering makes
more sense.

Second I think this patch is incorrect for XFS - XFS uses ->update_time
to set the time stampst in the dinode.  These two need to be coherent
as we can write out a dirty inode any time, so it needs to have the
timestamp uptodate.

Third update_time now calls mark_inode_dirty unconditionally, while
previously it wasn't called when ->update_time was set.  At least
for XFS that's a major change in behavior as XFS never used VFS dirty
tracking for metadata updates.