io_uring/sqpoll: zero sqd->thread on tctx errors

Syzkeller reports:

BUG: KASAN: slab-use-after-free in thread_group_cputime+0x409/0x700 kernel/sched/cputime.c:341
Read of size 8 at addr ffff88803578c510 by task syz.2.3223/27552
 Call Trace:
  <TASK>
  ...
  kasan_report+0x143/0x180 mm/kasan/report.c:602
  thread_group_cputime+0x409/0x700 kernel/sched/cputime.c:341
  thread_group_cputime_adjusted+0xa6/0x340 kernel/sched/cputime.c:639
  getrusage+0x1000/0x1340 kernel/sys.c:1863
  io_uring_show_fdinfo+0xdfe/0x1770 io_uring/fdinfo.c:197
  seq_show+0x608/0x770 fs/proc/fd.c:68
  ...

That's due to sqd->task not being cleared properly in cases where
SQPOLL task tctx setup fails, which can essentially only happen with
fault injection to insert allocation errors.

Cc: stable@vger.kernel.org
Fixes: 1251d202 ("io_uring/sqpoll: early exit thread if task_context wasn't allocated")
Reported-by:  <syzbot+3d92cfcfa84070b0a470@syzkaller.appspotmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/efc7ec7010784463b2e7466d7b5c02c2cb381635.1736519461.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>