block/nbd: micro-optimization in nbd request completion

Add in-flight cmds to the tail. That way while searching
(during request completion),we will always get a hit on the
first element.

Signed-off-by: Chetan Loke <loke.chetan@gmail.com>
Acked-by:  <Paul.Clements@steeleye.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>