datapath: rhel: Move RHEL OVS hook registration to netdev_rx_handler_register() backport

Moves the registration of the RHEL specific OVS hook to the compat
backport of netdev_rx_handler_register(). This moves the hook
unregistration from the RCU callback to the netdev_destroy()
callback directly.

This is purely cosmetic though, the RHEL hook is only used if the
IFF_OVS_DATAPATH flag is present which was removed under RTNL
protection before the RCU callback.

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