ppp: release rtnl mutex when interface creation fails

Add missing rtnl_unlock() in the error path of ppp_create_interface().

Fixes: 58a89ecaca53 ("ppp: fix lockdep splat in ppp_dev_uninit()")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index e8a5936..d61da9ec 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2816,6 +2816,7 @@
 
 out2:
 	mutex_unlock(&pn->all_ppp_mutex);
+	rtnl_unlock();
 	free_netdev(dev);
 out1:
 	*retp = ret;