memory: set cgroupv2 memory.high to ~99.6% of memory.max hard-limit

memory: set cgroupv2 memory.high to ~99.6% of memory.max hard-limit

cgroup memory usage is limited by the hard 'max' limit (OOM-killer
enforced) and the soft 'high' limit (cgroup processes get throttled
and put under heavy reclaim pressure). Set the latter high limit to
1016/1024 (~99.2%) of the 'max' hard limit, this scales with CT
memory allocations, & gives a decent 2^x based rest for 2^y memory
config which is still quite near the upper bound – clamp the maximum
gap between high and max at 128 MiB to avoid that huge container pay
quite an high amount of absolute cost.

A few example for differences between max & high for a few mem sizes:
- 2 MiB lower for 256 MiB max
- 16 MiB lower for 2 GiB max
- 128 MiB for 16 GiB and above

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>