epair: Fix panic on unload

The VNET_SYSUNINIT() callback is executed after the MOD_UNLOAD. That means
that netisr_unregister() has already been called when
netisr_unregister_vnet() gets calls, leading to an assertion failure.

Restore the expected order of operations by performing everything that
was done in MOD_UNLOAD to a SYSUNINIT() (that will be called after the
VNET_SYSUNINIT()).

Differential Revision: https://reviews.freebsd.org/D12771