io_uring: track link's head and tail during submit

Explicitly save not only a link's head in io_submit_sqe[s]() but the
tail as well. That's in preparation for keeping linked requests in a
singly linked list.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>