Several device drivers call if_alloc() and then do further checks and

will cal if_free() in case of conflict, error, ..
if_free() however sets the VNET instance from the ifp->if_vnet which
was not yet initialized but would only in if_attach(). Fix this by
setting the curvnet from where we allocate the interface in if_alloc().
if_attach() will later overwrite this as needed. We do not set the home_vnet
early on as we only want to prevent the if_free() panic but not change any
of the other housekeeping, e.g., triggered through ifioctl()s.

Reviewed by:	brooks
Approved by:	re (gjb)
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D7010

sys/net/if.c

@@ -455,6 +455,9 @@ if_alloc(u_char type)
 	ifp->if_index = idx;
 	ifp->if_type = type;
 	ifp->if_alloctype = type;
+#ifdef VIMAGE
+	ifp->if_vnet = curvnet;
+#endif
 	if (if_com_alloc[type] != NULL) {
 		ifp->if_l2com = if_com_alloc[type](type, ifp);
 		if (ifp->if_l2com == NULL) {