ppp: use the correct function to check if a netdev name is in use

A new helper to detect if a net device name is in use was added. Use it
here as the return reference from __dev_get_by_name was discarded.
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -1161,7 +1161,7 @@ static int ppp_unit_register(struct ppp *ppp, int unit, bool ifname_is_set)
 		if (!ifname_is_set) {
 			while (1) {
 				snprintf(ppp->dev->name, IFNAMSIZ, "ppp%i", ret);
-				if (!__dev_get_by_name(ppp->ppp_net, ppp->dev->name))
+				if (!netdev_name_in_use(ppp->ppp_net, ppp->dev->name))
 					break;
 				unit_put(&pn->units_idr, ret);
 				ret = unit_get(&pn->units_idr, ppp, ret + 1);