datapath: Make 'struct work_struct' consistent with kernel definition.

datapath: Make 'struct work_struct' consistent with kernel definition.

From kernel 3.4 netdevice structure has delayed_work in
net_device->pm_qos_req. delayed_work needs work_struct definition.
OVS has its own workq implementation which redefines work_struct.
So we need to make it consistent with work_struct defined
in kernel workqueue.h to have correct net_device definition.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>