datapath: Backport workqueue functions.

datapath: Backport workqueue functions.

An upcoming commit will use some workqueue functions that weren't
available on earlier kernels, so this backports those functions.
The backporting uses timers instead of delayed work queues because
the earlier versions of work queues have some unsafe corner cases.
In addition, this removes some unused work queue backporting code
that is no longer used because it is potentially unsafe.

Note that this commit changes the behavior of work queues: normally
they run in process context but the backported version runs in
softirq context.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Reviewed-by: Ben Pfaff <blp@nicira.com>