block: use for_each_thread() in sys_ioprio_set()/sys_ioprio_get()

IOPRIO_WHO_USER case in sys_ioprio_set()/sys_ioprio_get() are using
while_each_thread(), which is unsafe under RCU lock according to commit
0c740d0afc3bff0a ("introduce for_each_thread() to replace the buggy
while_each_thread()").  Use for_each_thread() (via
for_each_process_thread()) which is safe under RCU lock.

Link: http://lkml.kernel.org/r/201702011947.DBD56740.OMVHOLOtSJFFFQ@I-love.SAKURA.ne.jp
Link: http://lkml.kernel.org/r/1486041779-4401-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

block/ioprio.c

@@ -122,14 +122,14 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
 			if (!user)
 				break;
 
-			do_each_thread(g, p) {
+			for_each_process_thread(g, p) {
 				if (!uid_eq(task_uid(p), uid) ||
 				    !task_pid_vnr(p))
 					continue;
 				ret = set_task_ioprio(p, ioprio);
 				if (ret)
 					goto free_uid;
-			} while_each_thread(g, p);
+			}
 free_uid:
 			if (who)
 				free_uid(user);
@@ -222,7 +222,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 			if (!user)
 				break;
 
-			do_each_thread(g, p) {
+			for_each_process_thread(g, p) {
 				if (!uid_eq(task_uid(p), user->uid) ||
 				    !task_pid_vnr(p))
 					continue;
@@ -233,7 +233,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 					ret = tmpio;
 				else
 					ret = ioprio_best(ret, tmpio);
-			} while_each_thread(g, p);
+			}
 
 			if (who)
 				free_uid(user);