drivers/net/wan/lapbether: Added needed_tailroom

The underlying Ethernet device may request necessary tailroom to be
allocated by setting needed_tailroom. This driver should also set
needed_tailroom to request the tailroom needed by the underlying
Ethernet device to be allocated.

Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>