librbd: use uint64 for return value of size()

librbd: use uint64 for return value of size()

size_t is not necessarily an alias of uint64_t, so template argument
deduction would fail if we use `std::min(m_object_map.size(),
prev_object_diff_state_size)`. so better off using the same type for lhs
and rhs.

Signed-off-by: Kefu Chai <kchai@redhat.com>