Kernel.Vmm: Attempt to address race conditions involving ClampRangeSize, CopySparseMemory, and TryWriteBacking (#3956)

* no

no

* Adjust locking strategy

Use a separate mutex for the initial error checks + GPU unmap instead of using the reader lock. Make sure all writers lock this separate mutex, and for those that don't perform GPU unmaps, lock the writer lock immediately too.

This gets around every race condition I've envisioned so far, and hopefully does the trick?

* Clang

* Always GPU unmap

GPU unmaps have logic built-in to only run on mapped areas.
Not sure if userfaultfd would work with this, but since that's already broken anyway, I'll let reviewers decide that.

Without doing this, I'd need to do an extra pass through VMAs to find what all needs to be GPU modified before I can unmap from GPU, then perform remaining unmap work. Especially for places like MapMemory, that's a lot of code bloat.

* Fixups

* Update memory.cpp

* Rename mutex

It's really just a mutex for the sole purpose of dealing with GPU unmaps, so unmap_mutex is a bit more fitting than transition_mutex