datapath: Use openvswitch_handle_frame hook in >=RHEL6.4 to live side by side with...

datapath: Use openvswitch_handle_frame hook in >=RHEL6.4 to live side by side with bridging

Due to the missing register rx_handler API in the kernel RHEL6 is
based on, the datapath currently falls back to using the bridging
hook with the consequence that bridging and OVS cannot be used in
parallel on any RHEL6 release.

For this purpose, >=RHEL6.4 releases provide a special rx frame hook
to be used by OVS. It captures frames at the same location in the
stack as the rx_handler would do in more recent kernel releases. In
order to store the vport pointer, the net_device's ax25_ptr field is
utilized under the assumption that an AX25 device will never be
attached to an OVS bridge.

Signed-off-by: Thomas Graf <tgraf@redhat.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>