bpf: Fix verifier assumptions about socket->sk

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix verifier assumptions about socket->sk

The verifier assumes that 'sk' field in 'struct socket' is valid

and non-NULL when 'socket' pointer itself is trusted and non-NULL.

That may not be the case when socket was just created and

passed to LSM socket_accept hook.

Fix this verifier assumption and adjust tests.