bpf: Test ref release issue in bpf_tcp_sock and bpf_sk_fullsock

Adding verifier tests to ensure the ptr returned from bpf_tcp_sock() and
bpf_sk_fullsock() cannot be accessed after bpf_sk_release() is called.
A few of the tests are derived from a reproducer test by Lorenz Bauer.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>