ovs: fix dp check in ovs_dp_reset_user_features

This fixes crash when userspace does "ovs-dpctl add-dp dev" where dev is
existing non-dp netdevice.

Introduced by:
commit 44da5ae5
"openvswitch: Drop user features if old user space attempted to create datapath"
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Jesse Gross <jesse@nicira.com>

net/openvswitch/datapath.c

@@ -1174,7 +1174,7 @@ static void ovs_dp_reset_user_features(struct sk_buff *skb, struct genl_info *in
 	struct datapath *dp;
 
 	dp = lookup_datapath(sock_net(skb->sk), info->userhdr, info->attrs);
-	if (!dp)
+	if (IS_ERR(dp))
 		return;
 
 	WARN(dp->user_features, "Dropping previously announced user features\n");