xfs: plumb in needed functions for range querying of various btrees

Plumb in the pieces (init_high_key, diff_two_keys) necessary to call
query_range on the inode space and block mapping btrees and to extract
raw btree records.  This will eventually be used by the inobt and bmbt
scrubbers.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>