runtime: the scavenger doesn't call MADV_NOHUGEPAGE like it used to #55328
Description
Because the Go runtime's background scavenger releases memory in 64 KiB chunks, the heuristic in mem_linux.go's implementation of sysUnusedOS never actually calls MADV_NOHUGEPAGE to break up huge pages that would otherwise keep a lot of memory around. The reason for this is that there's a heuristic preventing the call on every sysUnusedOS to avoid creating too many VMAs (Linux has a low limit).
The result of all this is that Linux may be keeping around huge pages transparently using up a lot more memory than intended, and reducing the effectiveness of the scavenger.
I think the fix for this is to mark all new heap memory as MADV_NOHUGEPAGE initially, and then only call it again in circumstances where we free memory that is likely to have had MADV_HUGEPAGE called on it (e.g. a large object that contains at least one aligned 2 MiB region will have this happen via sysUsed). However, this might not be enough, and needs some more investigation.
CC @golang/runtime