bpf: Add test for syscall on fd array/htab lookup

Checks are added to the existing sockex3 and test_map_in_map test.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>