workqueue: Allow retrieval of current task's work struct

Introduce a helper to retrieve the current task's work struct if it is
a workqueue worker.

This allows us to fix a long-standing deadlock in several DRM drivers
wherein the ->runtime_suspend callback waits for a specific worker to
finish and that worker in turn calls a function which waits for runtime
suspend to finish.  That function is invoked from multiple call sites
and waiting for runtime suspend to finish is the correct thing to do
except if it's executing in the context of the worker.

Cc: Lai Jiangshan <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Ben Skeggs <[email protected]>
Cc: Alex Deucher <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Signed-off-by: Lukas Wunner <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/2d8f603074131eb87e588d2b803a71765bd3a2fd.1518338788.git.lukas@wunner.de

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