io_uring: alloc ->io in io_req_defer_prep()

Every call to io_req_defer_prep() is prepended with allocating ->io,
just do that in the function. And while we're at it, mark error paths
with unlikey and replace "if (ret < 0)" with "if (ret)".

There is only one change in the observable behaviour, that's instead of
killing the head request right away on error, it postpones it until the
link is assembled, that looks more preferable.

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