mm/page_alloc.c: fix type information of memoryless node

For a memoryless node, the output of get_pfn_range_for_nid are all zero.
It will display mem from 0 to -1.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
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>