tcp: Improve handling of fallback if socket pool is empty on new splice

When creating a new spliced connection, we need to get a socket in the
other ns from the originating one.  To avoid excessive ns switches we
usually get these from a pool refilled on a timer.  However, if the pool
runs out we need a fallback.  Currently that's done by passing -1 as the
socket to tcp_splice_connnect() and running it in the target ns.

This means that tcp_splice_connect() itself needs to have different cases
depending on whether it's given an existing socket or not, which is
a separate concern from what it's mostly doing.  We change it to require
a suitable open socket to be passed in, and ensuring in the caller that we
have one.

This requires adding the fallback paths to the caller, tcp_splice_new().
We use slightly different approaches for a socket in the init ns versus the
guest ns.

This also means that we no longer need to run tcp_splice_connect() itself
in the guest ns, which allows us to remove a bunch of boilerplate code.

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

tcp_conn.h

@@ -193,6 +193,7 @@ 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[]);
+int tcp_conn_new_sock(const struct ctx *c, sa_family_t af);
 void tcp_sock_refill_pool(const struct ctx *c, int pool[], int af);
 void tcp_splice_refill(const struct ctx *c);