openvswitch: checking wrong variable in queue_userspace_packet()

"skb" is non-NULL here, for example we dereference it in skb_clone().
The intent was to test "nskb" which was just set.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>