Fix panic during lagg destruction with simultaneous status check

[asomers]

Summary

Fix panic during lagg destruction with simultaneous status check

If you run "ifconfig lagg0 destroy" and "ifconfig lagg0" at the same time a
page fault may result. The first process will destroy ifp->if_lagg in
lagg_clone_destroy (called by if_clone_destroy). Then the second process
will observe that ifp->if_lagg is NULL at the top of lagg_port_ioctl and
goto fallback: where it will promptly dereference ifp->if_lagg anyway.

The solution is to repeat the NULL check for ifp->if_lagg

Test Plan

I have a kyua test script that repeately destroys a lagg while checking its
status. But I can't commit it yet because it panics too frequently.