ANDROID: KVM: arm64: Coalesce host stage2 entries on ownership reclaim

This optimization allows us to re-create higher order block mappings in
the host stage2 pagetables after we teardown a guest VM.
When the host reclaims ownership during guest teardown, the page table
walker drops the refcount of the counted entries and clears out
unreferenced entries (refcount == 1). Clearing out the entry installs a
zero PTE. When the host stage2 receives a data abort because there is no
mapping associated, it will try to create the largest possible block
mapping from the founded leaf entry.
With the current patch, we increase the chances of finding a leaf entry
that has level < 3 if the requested region comes from a reclaimed torned
down VM memory. This has the advantage of reducing the TLB pressure at
host stage2.

To increase the coalescing chances, we modify the way we refcount page
table descriptors for host stage2:
- non-zero invalid PTEs
- any of the reserved-high bits(58-55) toogled
- non-default attribute mappings
- page table descriptors

Bug: 222044487
Test: dump the host stage2 pagetables and view the mapping
Signed-off-by: Sebastian Ene <sebastianene@google.com>
Change-Id: I90ff4ec2185e9a76d7ad17e77ef9bdd8ce3e8698