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.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D28530