mm/memory.c: make apply_to_page_range() more robust

Arm and arm64 used to trigger this BUG_ON() - this has now been fixed.

But a WARN_ON() here is sufficient to catch future buggy callers.

Signed-off-by: Mika Penttilä <mika.penttila@nextfour.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>