task_work: unconditionally run task_work from get_signal()

Song reported a boot regression in a kvm image with 5.11-rc, and bisected
it down to the below patch. Debugging this issue, turns out that the boot
stalled when a task is waiting on a pipe being released. As we no longer
run task_work from get_signal() unless it's queued with TWA_SIGNAL, the
task goes idle without running the task_work. This prevents ->release()
from being called on the pipe, which another boot task is waiting on.

For now, re-instate the unconditional task_work run from get_signal().
For 5.12, we'll collapse TWA_RESUME and TWA_SIGNAL, as it no longer
makes sense to have a distinction between the two. This will turn
task_work notification into a simple boolean, whether to notify or not.

Fixes: 98b89b6 ("signal: kill JOBCTL_TASK_WORK")
Reported-by: Song Liu <songliubraving@fb.com>
Tested-by: John Stultz <john.stultz@linaro.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM/Clang version 11.0.1
Signed-off-by: Jens Axboe <axboe@kernel.dk>

kernel/signal.c

@@ -2550,6 +2550,9 @@ bool get_signal(struct ksignal *ksig)
 	struct signal_struct *signal = current->signal;
 	int signr;
 
+	if (unlikely(current->task_works))
+		task_work_run();
+
 	/*
 	 * For non-generic architectures, check for TIF_NOTIFY_SIGNAL so
 	 * that the arch handlers don't all have to do it. If we get here