rados.py: correct some C types

trunc was getting size_t instead of uint64_t, leading to bad results
in 32-bit environments. Explicitly cast to the desired type
everywhere, so it's clear the correct type is being used.

Fixes: #5233
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #346 from ceph/wip-5233

rados.py: correct some C types