mm/rmap: silence VM_WARN_ON_FOLIO() in __folio_rmap_sanity_checks()

Unfortunately, vm_insert_page() and friends and up passing
driver-allocated folios into folio_add_file_rmap_pte() using
insert_page_into_pte_locked().

While these driver-allocated folios can be compound pages (large folios),
they are not proper "rmappable" folios.

In these VM_MIXEDMAP VMAs, there isn't really the concept of a reverse
mapping, so long-term, we should clean that up and not call into rmap
code.

For the time being, document how we can end up in rmap code with large
folios that are not marked rmappable.

Link: https://lkml.kernel.org/r/793c5cee-d5fc-4eb1-86a2-39e05686233d@redhat.com
Fixes: 68f03208

 ("mm/rmap: convert folio_add_file_rmap_range() into folio_add_file_rmap_[pte|ptes|pmd]()")
Reported-by:  <syzbot+50ef73537bbc393a25bb@syzkaller.appspotmail.com>
Closes: https://lkml.kernel.org/r/000000000000014174060e09316e@google.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yin Fengwei <fengwei.yin@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>