object cache: fix a wrong conversion assignment

If we assign the integer to a wider one, the width of that integer isn't
automatically converted and then we'll get a partial valume.

This fix following problem:

sheep -w size=1T # which will overflow a 32bit integer
dog vdi cache info # will get a wrong value

Reviewed-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>