vmxnet3: use initialized skb pointer to set hash

The recent conversion to skb_set_hash() was incorrect. 'skb' is
uninitialized at this point.

Fixes: 0b680703

 ('net: vmxnet3 calls skb_set_hash')
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Acked-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>