We currently rely on the kvm_is_transparent_hugepage() helper to
discover whether a given page has the potential to be mapped as
a block mapping.

However, this API doesn't really give un everything we want:
- we don't get the size: this is not crucial today as we only
  support PMD-sized THPs, but we'd like to have larger sizes
  in the future
- we're the only user left of the API, and there is a will
  to remove it altogether

To address the above, implement a simple walker using the existing
page table infrastructure, and plumb it into transparent_hugepage_adjust().
No new page sizes are supported in the process.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20210726153552.1535838-3-maz@kernel.org