net/sched: cls_flower: Mandate mask when matching on flags

When matching on flags, we should require the user to provide the
mask and avoid using an all-ones mask. Not doing so causes matching
on flags provided w.o mask to hit on the value being unset for all
flags, which may not what the user wanted to happen.

Fixes: faa3ffce ('net/sched: cls_flower: Add support for matching on flags')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Paul Blakey <paulb@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>