xfs: clean up xfs_buftarg

Clean up the xfs_buftarg structure a bit:
- remove bt_bsize which is never used
- replace bt_sshift with bt_ssize; we only ever shift it back

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>