xfs: refactor btree node scrubbing

Break up xchk_da_btree_entry and handle looking up leaf node entries
in the attr / dir callbacks, so that only the generic node handling
is left in the common core code.  Note that the checks for the crc
enabled blocks are removed, as the scrubbing code already remaps the
magic numbers earlier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>