nvmet: always initialize cqe.result

The spec doesn't mandate that the first two double words (aka results)
for the command queue entry need to be set to 0 when they are not
used (not specified). Though, the target implemention returns 0 for TCP
and FC but not for RDMA.

Let's make RDMA behave the same and thus explicitly initializing the
result field. This prevents leaking any data from the stack.

Signed-off-by: Daniel Wagner <dwagner@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>