if: avoid interface destroy race
When we destroy an interface while the jail containing it is being
destroyed we risk seeing a race between if_vmove() and the destruction
code, which results in us trying to move a destroyed interface.
Protect against this by using the ifnet_detach_sxlock to also covert
if_vmove() (and not just detach).
(cherry picked from commit 868bf82153e8ff22f09a8860c872149e0fb6bdef)