selftests/bpf: add test for bogus operations on pointers

Tests non-add/sub operations (AND, LSH) on pointers decaying them to
 unknown scalars.
Also tests that a pkt_ptr add which could potentially overflow is rejected
 (find_good_pkt_pointers ignores it and doesn't give us any reg->range).

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>