TODO: update

Drop resolved kdbus issues.
bus: fix memfd-cache regarding memfd offsets

We must restore part->mmap_begin when poping memfds from the memfd-cache.
We rely on the memfds to be unsealed, so we can be sure that we own the
whole FD. Therefore, simply set part->mmap_begin to the same as
part->data.

This fixes test-bus-kernel-benchmark.
bus: zero cmd_free before passing to ioctl

Make sure the whole cmd_free object is zeroed before passing it into the
kernel. This makes valgrind happy and makes us future proof.