ntb_netdev: correct skb leak

If ntb_netdev is unable to pass a new skb to the ntb transport for
future rx packets, it should free the newly alloc'ed skb in the error
case.  Found by Kernel memory leak detector.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>