netpoll: Fix device name check in netpoll_setup()

Apparently netpoll_setup() assumes that netpoll.dev_name is a pointer
when checking if the device name is set:

if (np->dev_name) {
  ...

However the field is a character array, therefore the condition always
yields true. Check instead whether the first byte of the array has a
non-zero value.
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -666,7 +666,7 @@ int netpoll_setup(struct netpoll *np)
 	int err;
 
 	rtnl_lock();
-	if (np->dev_name) {
+	if (np->dev_name[0]) {
 		struct net *net = current->nsproxy->net_ns;
 		ndev = __dev_get_by_name(net, np->dev_name);
 	}