net: Fix TOCTOU issue in sk_is_readable()

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

net: Fix TOCTOU issue in sk_is_readable()

sk->sk_prot->sock_is_readable is a valid function pointer when sk resides

in a sockmap. After the last sk_psock_put() (which usually happens when

socket is removed from sockmap), sk->sk_prot gets restored and

sk->sk_prot->sock_is_readable becomes NULL.

This makes sk_is_readable() racy, if the value of sk->sk_prot is reloaded

after the initial check. Which in turn may lead to a null pointer

dereference.

Ensure the function pointer does not turn NULL after the check.