tcp: Disable optimisations for tcp_hash()

I'm not sure if we're breaking some aliasing rule here, but with gcc
12.2.1 on x86_64 and -flto, the siphash_20b() call in tcp_hash()
doesn't see the connection address -- it gets all zeroes instead.

Fix this temporarily by disabling optimisations for this tcp_hash().

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tcp.c

@@ -1185,6 +1185,9 @@ static int tcp_hash_match(const struct tcp_tap_conn *conn,
 #if TCP_HASH_NOINLINE
 __attribute__((__noinline__))	/* See comment in Makefile */
 #endif
+__attribute__((optimize("O0")))	/* TODO: with -O2 and -flto on gcc 12.2,
+				 * siphash_20b() doesn't see 'addr', why?
+				 */
 static unsigned int tcp_hash(const struct ctx *c, const union inany_addr *addr,
 			     in_port_t tap_port, in_port_t sock_port)
 {