SLOW_WORK: Add support for cancellation of slow work

Add support for cancellation of queued slow work and delayed slow work items.
The cancellation functions will wait for items that are pending or undergoing
execution to be discarded by the slow work facility.

Attempting to enqueue work that is in the process of being cancelled will
result in ECANCELED.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>