mnl: Use mnl_socket_open() insatead of mnl_socket_open2()

mnl_socket_open2 is not yet available on e.g. Debian 8.  Furthermore,
osmo_fd_register() will set the CLOEXEC flag on every file descriptor
anyway, so there is no benefit from using mnl_socket_open2() at all.

Change-Id: I0b37ffa148ff0c0a22281b490820353f5fef00eb
diff --git a/src/mnl.c b/src/mnl.c
index 13d9c9e..a7d3511 100644
--- a/src/mnl.c
+++ b/src/mnl.c
@@ -74,7 +74,7 @@
 
 	olm->priv = priv;
 	olm->mnl_cb = mnl_cb;
-	olm->mnls = mnl_socket_open2(bus, SOCK_CLOEXEC);
+	olm->mnls = mnl_socket_open(bus);
 	if (!olm->mnls) {
 		LOGP(DLGLOBAL, LOGL_ERROR, "Error creating netlink socket for bus %d: %s\n",
 			bus, strerror(errno));