bpf, selftest: test {rd, wr}only flags and direct value access

Extend test_verifier with various test cases around the two kernel
extensions, that is, {rd,wr}only map support as well as direct map
value access. All passing, one skipped due to xskmap not present
on test machine:

  # ./test_verifier
  [...]
  #948/p XDP pkt read, pkt_meta' <= pkt_data, bad access 1 OK
  #949/p XDP pkt read, pkt_meta' <= pkt_data, bad access 2 OK
  #950/p XDP pkt read, pkt_data <= pkt_meta', good access OK
  #951/p XDP pkt read, pkt_data <= pkt_meta', bad access 1 OK
  #952/p XDP pkt read, pkt_data <= pkt_meta', bad access 2 OK
  Summary: 1410 PASSED, 1 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>