xfs: switch to multigrain timestamps

xfs: switch to multigrain timestamps

Source kernel commit: 1cf7e834a6fb84de9d1e038d6cf4c5bd0d202ffa

Enable multigrain timestamps, which should ensure that there is an
apparent change to the timestamp whenever it has been written after
being actively observed via getattr.

Also, anytime the mtime changes, the ctime must also change, and those
are now the only two options for xfs_trans_ichgtime. Have that function
unconditionally bump the ctime, and ASSERT that XFS_ICHGTIME_CHG is
always set.

Finally, stop setting STATX_CHANGE_COOKIE in getattr, since the ctime
should give us better semantics now.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org> # documentation bits
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241002-mgtime-v10-9-d1c4717f5284@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>