KVM x86 MMU changes for 6.12:

 - Overhaul the "unprotect and retry" logic to more precisely identify cases
   where retrying is actually helpful, and to harden all retry paths against
   putting the guest into an infinite retry loop.

 - Add support for yielding, e.g. to honor NEED_RESCHED, when zapping rmaps in
   the shadow MMU.

 - Refactor pieces of the shadow MMU related to aging SPTEs in prepartion for
   adding MGLRU support in KVM.

 - Misc cleanups