net: fixing a memory leak in if_deregister_com_alloc()

Drain the callbacks upon if_deregister_com_alloc() such that the
if_com_free[type] won't be nullified before if_destroy().

Taking fwip(4) as an example, before this fix, kldunload if_fwip will
go through the following:

1. fwip_detach()
2. if_free() -> schedule if_destroy() through NET_EPOCH_CALL
3. fwip_detach() returns
4. firewire_modevent(MOD_UNLOAD) -> if_deregister_com_alloc()
5. kernel complains about:

Warning: memory type fw_com leaked memory on destroy (1 allocations, 64 bytes leaked).

6. EPOCH runs if_destroy() -> if_free_internal()i

By this time, if_com_free[if_alloctype] is NULL since it's already
nullified by if_deregister_com_alloc(); hence, firewire_free() won't
have a chance to release the allocated fw_com.

Reviewed by: hselasky, glebius
MFC after: 2 weeks