ax25: Quick fix for making sure unaccepted sockets get destroyed.

Since we reverted 30902dc3 ("ax25: Fix
std timer socket destroy handling.") we have to put some kind of fix
in to cure the issue whereby unaccepted connections do not get destroyed.

The approach used here is from Tihomir Heidelberg - 9a4gl

Signed-off-by: David S. Miller <davem@davemloft.net>