io_uring: don't use {test,clear}_tsk_thread_flag() for current

Linus correctly points out that this is both unnecessary and generates
much worse code on some archs as going from current to thread_info is
actually backwards - and obviously just wasteful, since the thread_info
is what we care about.

Since io_uring only operates on current for these operations, just use
test_thread_flag() instead. For io-wq, we can further simplify and use
tracehook_notify_signal() to handle the TIF_NOTIFY_SIGNAL work and clear
the flag. The latter isn't an actual bug right now, but it may very well
be in the future if we place other work items under TIF_NOTIFY_SIGNAL.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/io-uring/CAHk-=wgYhNck33YHKZ14mFB5MzTTk8gqXHcfj=RWTAXKwgQJgg@mail.gmail.com/Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -388,11 +388,9 @@ static struct io_wq_work *io_get_next_work(struct io_wqe *wqe)
 
 static bool io_flush_signals(void)
 {
-	if (unlikely(test_tsk_thread_flag(current, TIF_NOTIFY_SIGNAL))) {
+	if (unlikely(test_thread_flag(TIF_NOTIFY_SIGNAL))) {
 		__set_current_state(TASK_RUNNING);
-		if (current->task_works)
-			task_work_run();
-		clear_tsk_thread_flag(current, TIF_NOTIFY_SIGNAL);
+		tracehook_notify_signal();
 		return true;
 	}
 	return false;

fs/io_uring.c

@@ -6873,7 +6873,7 @@ static int io_run_task_work_sig(void)
 		return 1;
 	if (!signal_pending(current))
 		return 0;
-	if (test_tsk_thread_flag(current, TIF_NOTIFY_SIGNAL))
+	if (test_thread_flag(TIF_NOTIFY_SIGNAL))
 		return -ERESTARTSYS;
 	return -EINTR;
 }