[PATCH] sys_ioprio_set: minor do_each_thread+break fix

From include/linux/sched.h:

         * Careful: do_each_thread/while_each_thread is a double loop so
         *          'break' will not work as expected - use goto instead.
         */

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Jens Axboe <axboe@suse.de>