bpf: export bpf_sock for BPF_PROG_TYPE_SOCK_OPS prog type

And let it use bpf_sk_storage_{get,delete} helpers to access socket
storage. Kernel context (struct bpf_sock_ops_kern) already has sk
member, so I just expose it to the BPF hooks. I use
PTR_TO_SOCKET_OR_NULL and return NULL in !is_fullsock case.

I also export bpf_tcp_sock to make it possible to access tcp socket stats.

Cc: Martin Lau <kafai@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>