xfs: introduce a common helper xfs_icluster_size_fsb

Introduce a common routine xfs_icluster_size_fsb() to calculate
and return the number of file system blocks per inode cluster.

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