erofs: move erofs_workgroup operations into zdata.c

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

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241021035323.3280682-2-hsiangkao@linux.alibaba.com