tcp: Split pool lookup from creating new sockets in tcp_conn_new_sock()

tcp_conn_new_sock() first looks for a socket in a pre-opened pool, then if
that's empty creates a new socket in the init namespace.  Both parts of
this are duplicated in other places: the pool lookup logic is duplicated in
tcp_splice_new(), and the socket opening logic is duplicated in
tcp_sock_refill_pool().

Split the function into separate parts so we can remove both these
duplications.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tcp_conn.h

@@ -192,6 +192,7 @@ void tcp_splice_conn_update(struct ctx *c, struct tcp_splice_conn *new);
 void tcp_table_compact(struct ctx *c, union tcp_conn *hole);
 void tcp_splice_destroy(struct ctx *c, union tcp_conn *conn_union);
 void tcp_splice_timer(struct ctx *c, union tcp_conn *conn_union);
+int tcp_conn_pool_sock(int pool[]);
 void tcp_sock_refill_pool(const struct ctx *c, int pool[], int af);
 void tcp_splice_refill(const struct ctx *c);