xfs: port xfs_ioc_start_commit to multigrain timestamps

xfs: port xfs_ioc_start_commit to multigrain timestamps

Take advantage of the multigrain timestamp APIs to ensure that nobody
can sneak in and write things to a file between starting a file update
operation and committing the results.  This should have been part of the
multigrain timestamp merge, but I forgot to fling it at jlayton when he
resubmitted the patchset due to developer bandwidth problems.

Cc: <stable@vger.kernel.org> # v6.13-rc1
Fixes: 4e40eff0b5737c ("fs: add infrastructure for multigrain timestamps")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>