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 44da5ae5fbea4686f667dc854e5ea16814e44c59
"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>
diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index e9a48ba..e42340d 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -1174,7 +1174,7 @@
 	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");