util: Always return -1 on error in sock_l4()

According to its doc comments, sock_l4() returns -1 on error.  It does,
except in one case where it returns -EIO.  Fix this inconsistency to match
the docs and always return -1.

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

util.c

@@ -141,7 +141,7 @@ int sock_l4(const struct ctx *c, int af, uint8_t proto,
 
 	if (fd > SOCKET_MAX) {
 		close(fd);
-		return -EIO;
+		return -1;
 	}
 
 	ref.r.s = fd;