selftests/bpf: don't try to access past MAX_PACKET_OFF in test_verifier

A number of selftests fell foul of the changed MAX_PACKET_OFF handling.
For instance, "direct packet access: test2" was potentially reading four
 bytes from pkt + 0xffff, which could take it past the verifier's limit,
 causing the program to be rejected (checks against pkt_end didn't give
 us any reg->range).
Increase the shifts by one so that R2 is now mask 0x7fff instead of
 mask 0xffff.

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