xfs: block map scrub should handle incore delalloc reservations

Enhance the block map scrubber to check delayed allocation reservations.
Though there are no physical space allocations to check, we do need to
make sure that the range of file offsets being mapped are correct, and
to bump the lastoff cursor so that key order checking works correctly.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>