Fix a race condition that affects pfsync interface deletion.
When a pfsync interface is being deleted, all its timeout handlers and
pfsync_send_dispatch() have to stop accessing the software context
before the context is freed. Ensure sufficient synchronization by
acquiring NET_LOCK() and clearing `pfsyncif' inside the critical
section in pfsync_clone_destroy(). When a timeout handler has entered
the critical section, it has to check `pfsyncif' and bail out if the
value is NULL. pfsync_send_dispatch() already does this check.
Issue reported and fix tested by Hrvoje Popovski.
OK mpi@ bluhm@