xfs: check btree block ownership with bnobt/rmapbt when scrubbing btree

When scanning a metadata btree block, cross-reference the block location
with the free space btree and the reverse mapping btree to ensure that
the rmapbt knows about the block and the bnobt does not.  Add a
mechanism to defer checks when we happen to be scanning the bnobt/rmapbt
itself because it's less efficient to repeatedly clone and destroy the
cursor.

This patch provides the framework to make btree block owner checks
happen; the actual meat will be added in subsequent patches.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>