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 validate
(and potentially modify) the operation.

Fixes: 3e9a9e25 ("mm: add a vmap_pfn function")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>