cls_flower: Fix comparing of old filter mask with new filter

We incorrectly compare the mask and the result is that we can't modify
an already existing rule.

Fix that by comparing correctly.

Fixes: 05cd271fd61a ("cls_flower: Support multiple masks per priority")
Reported-by: Vlad Buslov <vladbu@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_flower.c

@@ -877,7 +877,7 @@ static int fl_check_assign_mask(struct cls_fl_head *head,
 			return PTR_ERR(newmask);
 
 		fnew->mask = newmask;
-	} else if (fold && fold->mask == fnew->mask) {
+	} else if (fold && fold->mask != fnew->mask) {
 		return -EINVAL;
 	}