netfilter: xt_socket: use sock_gen_put()

TCP listener refactoring, part 7 :

Use sock_gen_put() instead of xt_socket_put_sk() for future
SYN_RECV support.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>