mm: tidy up set_ptes definition

Now that all architectures are converted, we can remove the PFN_PTE_SHIFT
ifdef and we can define set_pte_at() unconditionally.

Link: https://lkml.kernel.org/r/20230802151406.3735276-33-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>