io_uring: fix task_work add use-after-free with SQPOLL

In the Linux kernel, the following vulnerability has been resolved:

io_uring: fix task_work add use-after-free with SQPOLL

For SQPOLL, the sqpoll thread runs task_work off its own list and
doesn't need to be told about it, so it can pop and complete a request
as soon as mpscq_push() has published it. If that was the last request
holding the ring alive, io_ring_exit_work() then frees the ctx, stops
the thread, and with it the tctx, while io_req_normal_work_add() is
still looking at both after the push:

BUG: KASAN: slab-use-after-free in io_req_normal_work_add+0x439/0x510
Read of size 4 at addr ff11000000ca0000 by task repro/55
(...)
BUG: KASAN: slab-use-after-free in queue_work_on+0x25/0x70
Write of size 8 at addr ff11000000c3bd00 by task repro/55

Hold an RCU read lock across the add, like io_req_local_work_add() already
does, and have io_ring_exit_work() wait for a grace period for SQPOLL rings
as well before freeing the ctx.