ionic: fix gathering of debug stats

Don't print stats for which we haven't reserved space as it can
cause nasty memory bashing and related bad behaviors.

Fixes: aa620993 ("ionic: pull per-q stats work out of queue loops")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>