log_cache: fix MemTracker name
log_cache: fix MemTracker name

This caused all of the MemTracker ids in the web ui to show up as
"log_cache::".

Change-Id: I9d482c9e242b271bd8e6a33ad4090cbdeafc1c63
Reviewed-on: http://gerrit.cloudera.org:8080/11654
Tested-by: Kudu Jenkins
Reviewed-by: Will Berkeley <c74686ab413a71926fc1bc9d16ebfa30e619336a@gmail.com>

InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu