io_uring: Fix use-after-free in io_sq_wq_submit_work()

when ctx->sqo_mm is zero, io_sq_wq_submit_work() frees 'req'
without deleting it from 'task_list'. After that, 'req' is
accessed in io_ring_ctx_wait_and_kill() which lead to
a use-after-free.

Signed-off-by: Guoyu Huang <hgy5945@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>