xfs: change bmap scrubber to store the previous mapping

Convert the inode data/attr/cow fork scrubber to remember the entire
previous mapping, not just the next expected offset.  No behavior
changes here, but this will enable some better checking in subsequent
patches.

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