selftests/bpf: Add C tests for kptr

This uses the __kptr and __kptr_ref macros as well, and tries to test
the stuff that is supposed to work, since we have negative tests in
test_verifier suite. Also include some code to test map-in-map support,
such that the inner_map_meta matches the kptr_off_tab of map added as
element.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220424214901.2743946-12-memxor@gmail.com