tcp: Move socket pool declarations around

tcp_splice.c has some explicit extern declarations to access the
socket pools.  This is pretty dangerous - if we changed the type of
these variables in tcp.c, we'd have tcp.c and tcp_splice.c using the
same memory in different ways with no compiler error.  So, move the
extern declarations to tcp_conn.h so they're visible to both tcp.c and
tcp_splice.c, but not the rest of pasta.

In fact the pools for the guest namespace are necessarily only used by
tcp_splice.c - we have no sockets on the guest side if we're not
splicing.  So move those declarations and the functions that deal
exclusively with them to tcp_splice.c

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

tcp.h

@@ -11,8 +11,6 @@
 #define TCP_CONN_INDEX_BITS		17	/* 128k */
 #define TCP_MAX_CONNS			(1 << TCP_CONN_INDEX_BITS)
 
-#define TCP_SOCK_POOL_SIZE		32
-
 struct ctx;
 
 void tcp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t events,