xfs: drain deferred work items when scrubbing

The design doc for XFS online fsck contains a long discussion of the
eventual consistency models in use for XFS metadata.  In that chapter,
we note that it is possible for scrub to collide with a chain of
deferred space metadata updates, and proposes a lightweight solution:
The use of a pending-intents counter so that scrub can wait for the
system to drain all chains.

This patchset implements that scrub drain.  The first patch implements
the basic mechanism, and the subsequent patches reduce the runtime
overhead by converting the implementation to use sloppy counters and
introducing jump labels to avoid walking into scrub hooks when it isn't
running.  This last paradigm repeats elsewhere in this megaseries.

v23.1: make intent items take an active ref to the perag structure and
       document why we bump and drop the intent counts when we do

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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