netpoll: fix an uninitialized variable

Fengguang reported:

   net/core/netpoll.c: In function 'netpoll_setup':
   net/core/netpoll.c:1049:6: warning: 'err' may be used uninitialized in this function [-Wmaybe-uninitialized]

in !CONFIG_IPV6 case, we may error out without initializing
'err'.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/netpoll.c

@@ -1140,6 +1140,7 @@ int netpoll_setup(struct netpoll *np)
 #else
 			np_err(np, "IPv6 is not supported %s, aborting\n",
 			       np->dev_name);
+			err = -EINVAL;
 			goto put;
 #endif
 		}