mm: Make mem_dump_obj() handle NULL and zero-sized pointers

This commit makes mem_dump_obj() call out NULL and zero-sized pointers
specially instead of classifying them as non-paged memory.

Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: <linux-mm@kvack.org>
Reported-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>