nbd: set rq->errors to actual error code

We've been relying on the block layer to assume rq->errors being set
translates into -EIO.  I noticed in testing that sometimes this isn't
true, and really there's not much of a reason to have a counter instead
of just using -EIO.  So set it properly so we don't leak random numbers
to unsuspecting victims.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>