xfs: use xfs_icluster_size_fsb in xfs_ifree_cluster

Use xfs_icluster_size_fsb() in xfs_ifree_cluster(), rename variable
ninodes to inodes_per_cluster, the latter is more meaningful.

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