io_uring: silence io_for_each_link() warning

Some tooling keep complaining about self assignment in
io_for_each_link(), the code is correct but still let's workaround it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f0de77b0b0f8309554ba6fba34327b7813bcc3ff.1648209006.git.asml.silence@gmail.com


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