staging: erofs: move shrink accounting inside the function

This patch moves the &erofs_global_shrink_cnt accounting
from the caller to erofs_workgroup_get().  It's cleaner and
it matches erofs_workgroup_put() better. No behavior change.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>