usercopy: Remove folio references from check_heap_object()

usercopy: Remove folio references from check_heap_object()

Because the pointer being checked may not lie within the first PAGE_SIZE
bytes of the object, we have to mark all pages as being LargeKmalloc.
We could use virt_to_head_page() instead, but that would pessimize
slab objects.  Once we move to memdescs properly, we'll tag each page
as being LargeKmalloc anyway, so this is more in keeping with how code
will be written in the future.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>