bpf: check NULL for sk_to_full_sk() return value

When req->rsk_listener is NULL, sk_to_full_sk() returns
NULL too, so we have to check its return value against
NULL here.

Fixes: 40304b2a ("bpf: BPF support for sock_ops")
Reported-by: David Ahern <dsahern@gmail.com>
Tested-by: David Ahern <dsahern@gmail.com>
Cc: Lawrence Brakmo <brakmo@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bpf-cgroup.h

@@ -85,7 +85,7 @@ int __cgroup_bpf_run_filter_sock_ops(struct sock *sk,
 	int __ret = 0;							       \
 	if (cgroup_bpf_enabled && (sock_ops)->sk) {	       \
 		typeof(sk) __sk = sk_to_full_sk((sock_ops)->sk);	       \
-		if (sk_fullsock(__sk))					       \
+		if (__sk && sk_fullsock(__sk))				       \
 			__ret = __cgroup_bpf_run_filter_sock_ops(__sk,	       \
 								 sock_ops,     \
 							 BPF_CGROUP_SOCK_OPS); \