Move related helpers into zdata.c as an intermediate step of getting
rid of `struct erofs_workgroup`, and rename:
erofs_workgroup_put => z_erofs_put_pcluster
erofs_workgroup_get => z_erofs_get_pcluster
erofs_try_to_release_workgroup => erofs_try_to_release_pcluster
erofs_shrink_workstation => z_erofs_shrink_scan
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>