io_uring: kill io_llist_xchg

io_llist_xchg is only used to set the list to NULL, which can also be
done with llist_del_all(). Use the latter and kill io_llist_xchg.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d6765112680d2e86a58b76166b7513391ff4e5d7.1729264960.git.asml.silence@gmail.com


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