ppp: Destroy the mutex when cleanup

BugLink: http://bugs.launchpad.net/bugs/1745052

[ Upstream commit f02b2320 ]

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: Gao Feng <gfree.wind@vip.163.com>
Acked-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

drivers/net/ppp/ppp_generic.c

@@ -942,6 +942,7 @@ 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);
 }