Block: use a freezable workqueue for disk-event polling

This patch (as1519) fixes a bug in the block layer's disk-events
polling.  The polling is done by a work routine queued on the
system_nrt_wq workqueue.  Since that workqueue isn't freezable, the
polling continues even in the middle of a system sleep transition.

Obviously, polling a suspended drive for media changes and such isn't
a good thing to do; in the case of USB mass-storage devices it can
lead to real problems requiring device resets and even re-enumeration.

The patch fixes things by creating a new system-wide, non-reentrant,
freezable workqueue and using it for disk-events polling.

Signed-off-by: Alan Stern <[email protected]>
CC: <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Acked-by: Rafael J. Wysocki <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

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