datapath: use rx_handler_data pointer

This adds compatibility with kernel changeset
"bridge: use rx_handler_data pointer to store net_bridge_port pointer"
which was added between 2.6.35 and 2.6.36-rc1.

With this change it is now safe to (attempt to) insert both bridge and
datapath with newer (>=2.6.36) kernels, although whichever is inserted
second will fail to initialise on the call to netdev_rx_handler_register()

Signed-off-by: Simon Horman <horms@verge.net.au>
[Jesse: fixed merge conflicts in vport-netdev.c and netdevice.h]
Signed-off-by: Jesse Gross <jesse@nicira.com>