nvmet: expose support for fabrics SQ flow control disable in treq

Technical Proposal introduces an indication for SQ flow control
disable support. Expose it since we are able to operate in this mode.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>