bpf: Check sk_fullsock() before returning from bpf_sk_lookup()

The BPF_FUNC_sk_lookup_xxx helpers return RET_PTR_TO_SOCKET_OR_NULL.
Meaning a fullsock ptr and its fullsock's fields in bpf_sock can be
accessed, e.g. type, protocol, mark and priority.
Some new helper, like bpf_sk_storage_get(), also expects
ARG_PTR_TO_SOCKET is a fullsock.

bpf_sk_lookup() currently calls sk_to_full_sk() before returning.
However, the ptr returned from sk_to_full_sk() is not guaranteed
to be a fullsock.  For example, it cannot get a fullsock if sk
is in TCP_TIME_WAIT.

This patch checks for sk_fullsock() before returning. If it is not
a fullsock, sock_gen_put() is called if needed and then returns NULL.

Fixes: 6acc9b43 ("bpf: Add helper to retrieve socket in BPF")
Cc: Joe Stringer <joe@isovalent.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Joe Stringer <joe@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>