bpf: add prog tag test case to bpf selftests

Add the test case used to compare the results from fdinfo with
af_alg's output on the tag. Tests are from min to max sized
programs, with and without maps included.

  # ./test_tag
  test_tag: OK (40945 tests)

Tested on x86_64 and s390x.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>