powerpc: hugetlb: Convert set_huge_pte_at() to take vma

In order to fix a bug, arm64 needs access to the vma inside it's
implementation of set_huge_pte_at(). Provide for this by converting the
mm parameter to be a vma. Any implementations that require the mm can
access it via vma->vm_mm.

This commit makes the required powerpc modifications. Separate commits
update the other arches and core code, before the actual bug is fixed in
arm64.

No behavioral changes intended.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>