block: fix error in handling dead task for ioprio setting

Don't combine the task exiting and "already have io_context" case, we
need to just abort if the task is marked as dead. Return -ESRCH, which
is the documented value for ioprio_set() if the specified task could not
be found.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by:  <syzbot+8836466a79f4175961b0@syzkaller.appspotmail.com>
Fixes: 5fc11eeb

 ("block: open code create_task_io_context in set_task_ioprio")
Signed-off-by: Jens Axboe <axboe@kernel.dk>