xfs: return the hash value of a leaf1 directory block

Modify the existing dir leafn lasthash function to enable us to
calculate the highest hash value of a leaf1 block.  This will be used by
the directory scrubbing code to check the sanity of hashes in leaf1
directory blocks.

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