nf: xt_socket: export the fancy sock finder code

The socket matching function has some nifty logic to get the struct sock
from the skb or from the connection tracker.
We export this so other xt_* can use it, similarly to ho how
xt_socket uses nf_tproxy_get_sock.

Change-Id: I11c58f59087e7f7ae09e4abd4b937cd3370fa2fd
Signed-off-by: JP Abgrall <jpa@google.com>