net: move interface removal notification up in if_detach_internal

This is needed to prevent having interfaces with ifp->if_addr == NULL
on bridge interfaces. Moving the notification event handlers up makes
sure the interfaces are removed before doing any more cleanup.

Sponsored by: Citrix Systems R&D
Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D598

net/if.c
 - Move interface removal notification up in if_detach_internal.