nvmet-tcp: implement C2HData SUCCESS optimization

TP 8000 says that the use of the SUCCESS flag depends on weather the
controller support disabling sq_head pointer updates. Given that we
support it by default, makes sense that we go the extra mile to actually
use the SUCCESS flag.

When we create the C2HData PDU header, we check if sqhd_disabled is set
on our queue, if so, we set the SUCCESS flag in the PDU header and
skip sending a completion response capsule.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Oliver Smith-Denny <osmithde@cisco.com>
Tested-by: Oliver Smith-Denny <osmithde@cisco.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>