mm/page_alloc: improve memmap_pages dbg msg

Make debug message more accurate.

Link: https://lkml.kernel.org/r/20210531091908.1738465-6-aisheng.dong@nxp.com


Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>