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: 4e40eff ("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>

Author: Darrick J. Wong

fs/xfs/xfs_exchrange.c

@@ -854,7 +854,7 @@ xfs_ioc_start_commit(
 	struct xfs_commit_range __user	*argp)
 {
 	struct xfs_commit_range		args = { };
-	struct timespec64		ts;
+	struct kstat			kstat = { };
 	struct xfs_commit_range_fresh	*kern_f;
 	struct xfs_commit_range_fresh	__user *user_f;
 	struct inode			*inode2 = file_inode(file);
@@ -871,12 +871,12 @@ xfs_ioc_start_commit(
 	memcpy(&kern_f->fsid, ip2->i_mount->m_fixedfsid, sizeof(xfs_fsid_t));
 
 	xfs_ilock(ip2, lockflags);
-	ts = inode_get_ctime(inode2);
-	kern_f->file2_ctime		= ts.tv_sec;
-	kern_f->file2_ctime_nsec	= ts.tv_nsec;
-	ts = inode_get_mtime(inode2);
-	kern_f->file2_mtime		= ts.tv_sec;
-	kern_f->file2_mtime_nsec	= ts.tv_nsec;
+	/* Force writing of a distinct ctime if any writes happen. */
+	fill_mg_cmtime(&kstat, STATX_CTIME | STATX_MTIME, inode2);
+	kern_f->file2_ctime		= kstat.ctime.tv_sec;
+	kern_f->file2_ctime_nsec	= kstat.ctime.tv_nsec;
+	kern_f->file2_mtime		= kstat.mtime.tv_sec;
+	kern_f->file2_mtime_nsec	= kstat.mtime.tv_nsec;
 	kern_f->file2_ino		= ip2->i_ino;
 	kern_f->file2_gen		= inode2->i_generation;
 	kern_f->magic			= XCR_FRESH_MAGIC;