Clear the internal table used by if_get(9) and sleep until all

Clear the internal table used by if_get(9) and sleep until all
remaining ifp references are released *without* the NET_LOCK().

It's safe to do so because the KERNEL_LOCK() serializes accesses
to ``if_map''.

More importantly this fix possible deadlocks between if_get() and
the NET_LOCK().  It is now possible to call them in whatever order.

ok visa@, dlg@