bpf, selftests: Add ringbuf memory type confusion test

Add two tests, one which asserts that ring buffer memory can be passed to
other helpers for populating its entry area, and another one where verifier
rejects different type of memory passed to bpf_ringbuf_submit().

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>