MB-8115: Don't count mem_size_counted in max_counted hash stat

Change-Id: I689eb1a83465424a850a68d8a7f5d1d57a3556f9
Reviewed-on: http://review.couchbase.org/25825
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>