workqueue: implement mod_delayed_work[_on]()

Workqueue was lacking a mechanism to modify the timeout of an already
pending delayed_work.  delayed_work users have been working around
this using several methods - using an explicit timer + work item,
messing directly with delayed_work->timer, and canceling before
re-queueing, all of which are error-prone and/or ugly.

This patch implements mod_delayed_work[_on]() which behaves similarly
to mod_timer() - if the delayed_work is idle, it's queued with the
given delay; otherwise, its timeout is modified to the new value.
Zero @delay guarantees immediate execution.

v2: Updated to reflect try_to_grab_pending() changes.  Now safe to be
    called from bh context.

Signed-off-by: Tejun Heo <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Ingo Molnar <[email protected]>

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