sunvnet: fix incorrect rcu_read_unlock() in vnet_start_xmit()

This patch removes an extra rcu_read_unlock() on an allocation failure
in vnet_skb_shape(). The needed rcu_read_unlock() is already done in
the out_dropped label.

Reported-by: Rashmi Narasimhan <rashmi.narasimhan@oracle.com>
Signed-off-by: David L Stevens <david.stevens@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>