mm: add nommu variant of vm_insert_pages()

An identical one exists for vm_insert_page(), add one for
vm_insert_pages() to avoid needing to check for CONFIG_MMU in code using
it.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>