io_uring/zcrx: recheck ifq on shutdown

io_ring_exit_work() checks ifq before shutting it down and guarantees
that the pointer is stable, but instead of relying on rather complicated
synchronisation recheck the ifq pointer inside.

Reported-by: Kees Bakker <kees@ijzerbout.nl>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/905e55c47235ab26377a735294f939f31d00ae53.1739934175.git.asml.silence@gmail.com


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