Widen ifnet_detach_sxlock coverage

Widen the ifnet_detach_sxlock to cover the entire vnet sysuninit code.
This ensures that we can't end up having the vnet_sysuninit free the UDP
pcb while the detach code is running and trying to purge the UDP pcb.

Approved by: re (gjb)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28530

(cherry picked from commit 6d2a10d96fb5d4ee42fd67b0b07a6d098db5d55a)
(cherry picked from commit 05530211165f59b8dabc02adaf26ea29c2726ebc)