ifnet: Initialize the address family dependent data region earlier

if_link_ifnet() adds the interface to the global network interface list,
and it is a natural synchronization point. With this change, any threads
that obtain the reference of the interface via ifunit(), ifunit_ref() etc.,
will be guaranteed to see the address family dependent data rightly.

The issue [1] reported by Mike Belanger also hints the potential race.

MFC note: this change depends on e64fe5ad3a, as calculating the max
IPv6 MTU through all the interfaces requires the current interface to
be added to the global network interface list firstly.

[1] https://lists.freebsd.org/archives/freebsd-net/2025-May/006817.html

Reviewed by:	glebius
MFC after:	1 month
MFC with:	e64fe5ad3a netinet6: Remove a set but not used global variable in6_maxmtu
Differential Revision:	https://reviews.freebsd.org/D49358

sys/net/if.c

@@ -946,11 +946,11 @@ if_attach_internal(struct ifnet *ifp, bool vmove)
 	}
 #endif
 
-	if_link_ifnet(ifp);
-
 	if (domain_init_status >= 2)
 		if_attachdomain1(ifp);
 
+	if_link_ifnet(ifp);
+
 	EVENTHANDLER_INVOKE(ifnet_arrival_event, ifp);
 	if (IS_DEFAULT_VNET(curvnet))
 		devctl_notify("IFNET", ifp->if_xname, "ATTACH", NULL);