mm: add default definition of set_ptes()

Most architectures can just define set_pte() and PFN_PTE_SHIFT to use this
definition.  It's also a handy spot to document the guarantees provided by
the MM.

Link: https://lkml.kernel.org/r/20230802151406.3735276-7-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Suggested-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>