datapath: check for rx handler register

datapath: check for rx handler register

Red Hat Enterprise Linux 6 has backported the netdev RX
handler facility so use the netdev_rx_handler_register as
an indicator.

The handler prototype changed between 2.6.36 and 2.6.39
since there could be backports in any stage, don't look
at the kernel version, but at the prototype.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>