block: restore the old set_task_ioprio() behaviour wrt PF_EXITING

PF_EXITING tasks were silently ignored before the below commits.
Continue doing so. Otherwise python-psutil tests fail:
  ERROR: psutil.tests.test_process.TestProcess.test_zombie_process
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/home/abuild/rpmbuild/BUILD/psutil-5.9.0/build/lib.linux-x86_64-3.9/psutil/_pslinux.py", line 1661, in wrapper
      return fun(self, *args, **kwargs)
    File "/home/abuild/rpmbuild/BUILD/psutil-5.9.0/build/lib.linux-x86_64-3.9/psutil/_pslinux.py", line 2133, in ionice_set
      return cext.proc_ioprio_set(self.pid, ioclass, value)
  ProcessLookupError: [Errno 3] No such process

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "/home/abuild/rpmbuild/BUILD/psutil-5.9.0/psutil/tests/test_process.py", line 1313, in test_zombie_process
      succeed_or_zombie_p_exc(fun)
    File "/home/abuild/rpmbuild/BUILD/psutil-5.9.0/psutil/tests/test_process.py", line 1288, in succeed_or_zombie_p_exc
      return fun()
    File "/home/abuild/rpmbuild/BUILD/psutil-5.9.0/build/lib.linux-x86_64-3.9/psutil/__init__.py", line 792, in ionice
      return self._proc.ionice_set(ioclass, value)
    File "/home/abuild/rpmbuild/BUILD/psutil-5.9.0/build/lib.linux-x86_64-3.9/psutil/_pslinux.py", line 1665, in wrapper
      raise NoSuchProcess(self.pid, self._name)
  psutil.NoSuchProcess: process no longer exists (pid=2057)

Cc: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Fixes: 5fc11ee (block: open code create_task_io_context in set_task_ioprio)
Fixes: a957b61 (block: fix error in handling dead task for ioprio setting)
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220328085928.7899-1-jslaby@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Jiri Slaby

block/blk-ioc.c

@@ -280,7 +280,6 @@ int set_task_ioprio(struct task_struct *task, int ioprio)
 
 		task_lock(task);
 		if (task->flags & PF_EXITING) {
-			err = -ESRCH;
 			kmem_cache_free(iocontext_cachep, ioc);
 			goto out;
 		}
@@ -292,7 +291,7 @@ int set_task_ioprio(struct task_struct *task, int ioprio)
 	task->io_context->ioprio = ioprio;
 out:
 	task_unlock(task);
-	return err;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(set_task_ioprio);