This change should allow to close a race between the code that frees
if_softc immediately after calling if_free(), such as
lagg_clone_destroy() for instance, and other accesses to the softc via a
reference to the interface that was obtained before the interface was
marked as dying.
The race is detailed in this bug report: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=234135