BUG#14485479: INSTALL AUDIT PLUGIN HANGS IF WE TRY TO

DISABLE AND ENABLED DURING DDL OPERATION

PROBLEM: Same thread trying to acquire the same mutex
         second time leads to hang/server crash.
         While [un]installing audit_log plugin
         a thread acquires the LOCK_plugin mutex
         and after successful initialization tries
         to write in mysql.plugin table. It holds
         this mutex for a long time. If some how
         plugin table is corrupted then a write to 
         plugin table will throw an error, thread try
         to log this error in the audit_log plugin,
         doing so it tries to acquire the mutex
         again and results is server hang/crash.

SOLUTION: Releasing the LOCK_plugin mutex before
          writing in mysql.plugin table. We dont
          need to hold this mutex as thread already
          acquired a TL_WRITE lock on mysql.plugin
          table.

mysql-test/include/plugin.defs

@@ -40,3 +40,4 @@ ha_blackhole       storage/blackhole  BLACKHOLE_PLUGIN
 ha_federated       storage/federated  FEDERATED_PLUGIN
 mypluglib          plugin/fulltext    SIMPLE_PARSER
 libdaemon_example  plugin/daemon_example DAEMONEXAMPLE
+adt_null           plugin/audit_null  AUDIT_NULL