xfs_db: make check work for sparse inodes

xfs_db: make check work for sparse inodes

Teach the inobt/finobt scanning functions how to deal with sparse
inode chunks well enough that we can pass the spot-check.  Should
fix the xfs/076 failures.

v2: Put the alignment checks back in, and for each individual chunk
of inodes hanging off an inobt record, parse each chunk separately.
There is no (future) guarantee that all the inodes in a sparse inobt
record will be contiguous.

Cc: Brian Foster <bfoster@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>