mm: print out memmap number only if it is not zero

Don't print the size of the zone's memmap array if it does not have one.

Impact: cleanup

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>