nvmet: set maxcmd to be per controller

This is a preparation for having a dynamic configuration of max queue
size for a controller. Make sure that the maxcmd field stays the same as
the MQES (+1) value as we do today.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Israel Rukshin <israelr@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>