MDEV-6749 - Deadlock between GRANT/REVOKE, SELECT FROM I_S.COLUMNS,

            SET slow_query_log and failed connection attempt

A very subtle though valid deadlock. Deadlock chain:
wrlock(LOCK_grant)    -> lock(acl_cache->lock) GRANT/REVOKE CREATE/DROP USER
lock(LOCK_open)       -> rdlock(LOCK_grant)    SELECT * FROM I_S.COLUMNS
wrlock(LOCK_logger)   -> lock(LOCK_open)       SET @@global.slow_query_log='ON'
lock(acl_cache->lock) -> rdlock(LOCK_logger)   Failed connection

Fixed by removing relationship between acl_cache->lock and LOCK_logger
during failed connection attempt.

sql/sql_acl.cc

@@ -9224,9 +9224,9 @@ bool acl_authenticate(THD *thd, uint connect_errors,
                                     mpvio.auth_info.authenticated_as, TRUE);
       if (!acl_proxy_user)
       {
+        mysql_mutex_unlock(&acl_cache->lock);
         if (!thd->is_error())
           login_failed_error(thd);
-        mysql_mutex_unlock(&acl_cache->lock);
         DBUG_RETURN(1);
       }
       acl_user= acl_proxy_user->copy(thd->mem_root);