staging: erofs: staticize erofs_shrink_count, erofs_shrink_scan

Move erofs_shrinker_info to utils.c and therefore
no need to globalize erofs_shrink_count and erofs_shrink_scan.

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