test_librbd: use correct type for varargs snap test

uint64_t is passed in, but int was extracted. This fails on 32-bit builds.

Fixes: #5220
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #344 from ceph/wip-5220

Reviewed-by: Sage Weil <sage@inktank.com>