Introduce movable pages for Hyper-V guests

From the start, the root-partition driver allocates, pins, and maps all
guest memory into the hypervisor at guest creation. This is simple: Linux
cannot move the pages, so the guest’s view in Linux and in Microsoft
Hypervisor never diverges.

However, this approach has major drawbacks:
- NUMA: affinity can’t be changed at runtime, so you can’t migrate guest memory closer to the CPUs running it → performance hit.
- Memory management: unused guest memory can’t be swapped out, compacted, or merged.
- Provisioning time: upfront allocation/pinning slows guest create/destroy.
- Overcommit: no memory overcommit on hosts with pinned-guest memory.

This series adds movable memory pages for Hyper-V child partitions. Guest
pages are no longer allocated upfront; they’re allocated and mapped into
the hypervisor on demand (i.e., when the guest touches a GFN that isn’t yet
backed by a host PFN).
When a page is moved, Linux no longer holds it and it is unmapped from the hypervisor.
As a result, Hyper-V guests behave like regular Linux processes, enabling standard Linux memory features to apply to guests.

Exceptions (still pinned):
  1. Encrypted guests (explicit).
  2 Guests with passthrough devices (implicitly pinned by the VFIO framework).

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Stanislav Kinsburskii (3):
      Drivers: hv: Rename a few memory region related functions for clarity
      Drivers: hv: Centralize guest memory region destruction in helper
      Drivers: hv: Add support for movable memory regions


 drivers/hv/Kconfig          |    1 
 drivers/hv/mshv_root.h      |    8 +
 drivers/hv/mshv_root_main.c |  448 +++++++++++++++++++++++++++++++++++++------
 3 files changed, 397 insertions(+), 60 deletions(-)