atomic_bitmap: use `usize::div_ceil()`

[00xc]

Summary of the PR

Instead of checking a division remainder, use usize::div_ceil(), which will round division up, adding an extra member to the bitmap when needed.

Since at the moment we unconditionally add 1 to map_size, this change results in allocating one less AtomicU64 in some cases.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [X] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• N/A All added/changed functionality has a corresponding unit/integration test.
• N/A All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• N/A Any newly added unsafe code is properly documented.

[00xc]

Hm, test toolchain is too old so it does not have a stabilized div_ceil().

[roypat]

Hi @00xc, To update the toolchain, you'd need to update the dockerfile definition in https://github.com/rust-vmm/rust-vmm-container, propage the new container version through to https://github.com/rust-vmm/rust-vmm-ci, and then update the rust-vmm-ci submodule.

[00xc]

Ping for reviewers