Linux 6.7 compat: zfs_setattr fix atime update

In db4fc55 I messed up and changed this bit of code to set the inode
atime to an uninitialised value, when actually it was just supposed to
loading the atime from the inode to be stored in the SA. This changes it
to what it should have been.

Ensure times change by the right amount Previously, we only checked
if the times changed at all, which missed a bug where the atime was
being set to an undefined value.

Now ensure the times change by two seconds (or thereabouts), ensuring
we catch cases where we set the time to something bonkers

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes openzfs#15762
Closes openzfs#15773

module/os/linux/zfs/zfs_vnops_os.c

@@ -2435,9 +2435,8 @@ zfs_setattr(znode_t *zp, vattr_t *vap, int flags, cred_t *cr, zidmap_t *mnt_ns)
 
 	if ((mask & ATTR_ATIME) || zp->z_atime_dirty) {
 		zp->z_atime_dirty = B_FALSE;
-		inode_timespec_t tmp_atime;
+		inode_timespec_t tmp_atime = zpl_inode_get_atime(ip);
 		ZFS_TIME_ENCODE(&tmp_atime, atime);
-		zpl_inode_set_atime_to_ts(ZTOI(zp), tmp_atime);
 		SA_ADD_BULK_ATTR(bulk, count, SA_ZPL_ATIME(zfsvfs), NULL,
 		    &atime, sizeof (atime));
 	}

tests/zfs-tests/cmd/ctime.c

@@ -362,12 +362,20 @@ main(void)
 			return (1);
 		}
 
-		if (t1 == t2) {
-			(void) fprintf(stderr, "%s: t1(%ld) == t2(%ld)\n",
+
+		/*
+		 * Ideally, time change would be exactly two seconds, but allow
+		 * a little slack in case of scheduling delays or similar.
+		 */
+		long delta = (long)t2 - (long)t1;
+		if (delta < 2 || delta > 4) {
+			(void) fprintf(stderr,
+			    "%s: BAD time change: t1(%ld), t2(%ld)\n",
 			    timetest_table[i].name, (long)t1, (long)t2);
 			return (1);
 		} else {
-			(void) fprintf(stderr, "%s: t1(%ld) != t2(%ld)\n",
+			(void) fprintf(stderr,
+			    "%s: good time change: t1(%ld), t2(%ld)\n",
 			    timetest_table[i].name, (long)t1, (long)t2);
 		}
 	}