nvme-fabrics: Ignore nr_io_queues option for discovery controllers

This removes a dependency on the order options are passed when creating
a fabrics controller.  With the old code, if "nr_io_queues" appears before
an "nqn" option specifying the discovery controller, then nr_io_queues
is overridden with zero.  If "nr_io_queues" appears after specifying the
discovery controller, then the nr_io_queues option is used to set the
number of queues, and the driver attempts to establish IO connections
to the discovery controller (which doesn't work).

It seems better to ignore (and warn about) the "nr_io_queues" option
if userspace has already asked to connect to the discovery controller.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Reviewed-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>