mm: Convert check_heap_object() to use struct slab

mm: Convert check_heap_object() to use struct slab

Ensure that we're not seeing a tail page inside __check_heap_object()
by converting to a slab instead of a page.  Take the opportunity to
mark the slab as const since we're not modifying it.  Also move the
declaration of __check_heap_object() to mm/slab.h so it's not
available to the wider kernel.

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