selftests/bpf: add a test for a pruning bug in the verifier

The test makes a read through a map value pointer, then considers pruning
 a branch where the register holds an adjusted map value pointer.  It
 should not prune, but currently it does.

Signed-off-by: Alexei Starovoitov <ast@fb.com>
[ecree@solarflare.com: added test-name and patch description]
Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>