nvmet: add helpers to set the result field for connect commands

The code to set the result field for the admin and I/O connect commands
is not only verbose and duplicated, but also violates the aliasing
rules as it accesses both the u16 and u32 members in the union.

Add a little helper to sort all that out.

Fixes: db1312dd ("nvmet: implement basic In-Band Authentication")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Hannes Reinecke <hare@suse.de>