Fix readonly check for vdev user properties

VDEV_PROP_USERPROP is equal do VDEV_PROP_INVAL and so is not a real
property.  That's why vdev_prop_readonly() does not work right for
it.  In particular it may declare all vdev user properties readonly
on FreeBSD.

Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.

module/zfs/vdev.c

@@ -5969,7 +5969,7 @@ vdev_prop_set(vdev_t *vd, nvlist_t *innvl, nvlist_t *outnvl)
 			goto end;
 		}
 
-		if (vdev_prop_readonly(prop)) {
+		if (prop != VDEV_PROP_USERPROP && vdev_prop_readonly(prop)) {
 			error = EROFS;
 			goto end;
 		}