ppp: Destroy the mutex when cleanup

The mutex_destroy only makes sense when enable DEBUG_MUTEX. For the
good readbility, it's better to invoke it in exit func when the init
func invokes mutex_init.
Signed-off-by: NGao Feng <gfree.wind@vip.163.com>
Acked-by: NGuillaume Nault <g.nault@alphalink.fr>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -960,6 +960,7 @@ static __net_exit void ppp_exit_net(struct net *net)
 	unregister_netdevice_many(&list);
 	rtnl_unlock();
 
+	mutex_destroy(&pn->all_ppp_mutex);
 	idr_destroy(&pn->units_idr);
 }