selftests/bpf: Add untrusted BTF write regression

Add a TCP congestion-control struct_ops load test for a write through a
BTF pointer produced by bpf_rdonly_cast().

The test expects the verifier to reject the program before the TCP CA
btf_struct_access callback can whitelist the tcp_sock field write.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>