nvme: add capability to connect to an administrative controller

Add capability to connect to an administrative controller by
preventing ioq creation for admin-controllers.

Add a nvme_admin_ctrl() to check if a controller's CNTRLTYPE indicates
that it is an administrative controller and override ctrl->queue_count to
1 for admin controllers, so that only the admin queue and no I/O queues
are created for an administrative controller.  This override is done in
nvme_init_ctrl_finish() after ctrl->cntrltype has been initialized in
nvme_init_identify() so nvme_admin_ctrl() will work correctly.
Doing this override in generic code (nvme_init_ctrl_finish) makes it
transport agnostic and will work properly for nvme/tcp as well as for
nvme/rdma.

Suggested-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Kamaljit Singh <kamaljit.singh1@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>