io_link_timeout_fn() removes REQ_F_LINK_TIMEOUT from the link head's
flags, it's not atomic and may race with what the head is doing.

If io_link_timeout_fn() doesn't clear the flag, as forced by this patch,
then it may happen that for "req -> link_timeout1 -> link_timeout2",
__io_kill_linked_timeout() would find link_timeout2 and try to cancel
it, so miscounting references. Teach it to ignore such double timeouts
by marking the active one with a new flag in io_prep_linked_timeout().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>