xfs: prepare reaping code for dynamic limits

xfs: prepare reaping code for dynamic limits

The online repair block reaping code employs static limits to decide if
it's time to roll the transaction or finish the deferred item chains to
avoid overflowing the scrub transaction's reservation.  However, the
use of static limits aren't great -- btree blocks are assumed to be
scattered around the AG and the buffers need to be invalidated, whereas
COW staging extents are usually contiguous and do not have buffers.  We
would like to configure the limits dynamically.

To get ready for this, reorganize struct xreap_state to store dynamic
limits, and add helpers to hide some of the details of how the limits
are enforced.  Also rename the "xreap roll" functions to include the
word "binval" because they only exist to decide when we should roll the
transaction to deal with buffer invalidations.

No functional changes intended here.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>