xfs: use xfs_icluster_size_fsb in xfs_bulkstat

Use xfs_icluster_size_fsb() in xfs_bulkstat(), make the related
variables more meaningful and remove an unused variable nimask
from it.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>