mm: vmalloc must set pte via arch code

It is bad practice to directly set pte entries within a pte table.
Instead all modifications must go through arch-provided helpers such as
set_pte_at() to give the arch code visibility and allow it to check
(and potentially modify) the operation.

Fixes: 3e9a9e25 ("mm: add a vmap_pfn function")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>