erofs: fix potential return value overflow of z_erofs_shrink_scan()

z_erofs_shrink_scan() could return small numbers due to the mistyped
`freed`.

Although I don't think it has any visible impact.

Fixes: 3883a79a ("staging: erofs: introduce VLE decompression support")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250114040058.459981-1-hsiangkao@linux.alibaba.com