iflib: Remove redundant VLAN events deregistration

[piotr.pietruszewski_intel.com]

Summary

r351152 introduced iflib_deregister() function calling
EVENTHANDLER_DEREGISTER() to unregister VLAN events. This patch removes
duplicate of EVENTHANDLER_DEREGISTER() calls placed in
iflib_device_deregister() as this function is now calling
iflib_deregister(). This is to avoid deregistering same event twice.

This patch also adds check in iflib_vlan_register() to prevent
registering VLAN while being in detach.

Patch co-authored by Krzysztof Galazka <[e-mail]>,
erj <[e-mail]> and Jacob Keller <[e-mail]>.

Signed-off-by: Piotr Pietruszewski <[e-mail]>

Test Plan

Before applying this patch, kernel panic should be observed after issuing:

for i in {1..10}; do kldunload if_ix; kldload if_ix; done

Usually, less than 10 cycles are needed to trigger the panic. After applying
the patch, panic should be gone.

[erj]

Bugzilla link: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=240610