bpf: NULL pointer check is not needed in BPF_CGROUP_RUN_PROG_INET_SOCK

sk is already allocated in inet_create/inet6_create, hence when
BPF_CGROUP_RUN_PROG_INET_SOCK is executed sk will never be NULL.

The logic is as bellow,
	sk = sk_alloc();
	if (!sk)
		goto out;
	BPF_CGROUP_RUN_PROG_INET_SOCK(sk);

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>