rpc: fix a bug where the reactor lock could get incorrectly unlocked
rpc: fix a bug where the reactor lock could get incorrectly unlocked

This bug was detected by TSAN under mt-rpc-test as "unlock an unlocked mutex"

Change-Id: I070736461511ab9cf98e9e11754b5a149200b56d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2087
Tested-by: jenkins
Reviewed-by: David Alves <33ea948168c114d220e0372a903be6ee60f6396e@cloudera.com>

cloudera/kudu,helifu/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,andrwng/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu