signal: unconditionally leave the frozen state in ptrace_stop()

Alex Xu reported a regression in strace, caused by the introduction of
the cgroup v2 freezer. The regression can be reproduced by stracing
the following simple program:

  #include <unistd.h>

  int main() {
      write(1, "a", 1);
      return 0;
  }

An attempt to run strace ./a.out leads to the infinite loop:
  [ pre-main omitted ]
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  write(1, "a", 1)                        = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  [ repeats forever ]

The problem occurs because the traced task leaves ptrace_stop()
(and the signal handling loop) with the frozen bit set. So let's
call cgroup_leave_frozen(true) unconditionally after sleeping
in ptrace_stop().

With this patch applied, strace works as expected:
  [ pre-main omitted ]
  write(1, "a", 1)                        = 1
  exit_group(0)                           = ?
  +++ exited with 0 +++

Reported-by: Alex Xu <alex_y_xu@yahoo.ca>
Fixes: 76f969e894 ("cgroup: cgroup v2 freezer")
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/signal.c

@@ -2113,6 +2113,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, kernel_siginfo_t
 		preempt_enable_no_resched();
 		cgroup_enter_frozen();
 		freezable_schedule();
+		cgroup_leave_frozen(true);
 	} else {
 		/*
 		 * By the time we got the lock, our tracer went away.