vfs: utimensat(): be consistent with utime() for immutable and append-only files

This patch fixes utimensat() to make its behavior consistent with that of utime()/utimes() when dealing with files marked immutable and append-only.

The current utimensat() implementation also returns EPERM if 'times' is non-NULL and the tv_nsec fields are both UTIME_NOW. For consistency, the

(times != NULL && times[0].tv_nsec == UTIME_NOW &&

                  times[1].tv_nsec == UTIME_NOW)

case should be treated like the traditional utimes() case where 'times' is NULL. That is, the call should succeed for a file marked append-only and should give the error EACCES if the file is marked as immutable.

The simple way to do this is to set 'times' to NULL
if (times[0].tv_nsec == UTIME_NOW && times[1].tv_nsec == UTIME_NOW).

This is also the natural approach, since POSIX.1 semantics consider the times == {{x, UTIME_NOW}, {y, UTIME_NOW}}
to be exactly equivalent to the case for
times == NULL.

(Thanks to Miklos for pointing this out.)

Patch 3 in this series relies on the simplification provided by this patch.

@@ -102,6 +102,10 @@
 	if (error)
 		goto dput_and_out;
 
+	if (times && times[0].tv_nsec == UTIME_NOW &&
+		     times[1].tv_nsec == UTIME_NOW)
+		times = NULL;
+
 	/* Don't worry, the checks are done in inode_change_ok() */
 	newattrs.ia_valid = ATTR_CTIME | ATTR_MTIME | ATTR_ATIME;
 	if (times) {