nvmet: make ctrl-id configurable

This patch adds a new target subsys attribute which allows user to
optionally specify target controller IDs which then used in the
nvmet_execute_identify_ctrl() to fill up the nvme_id_ctrl structure.

For example, when using a cluster setup with two nodes, with a dual
ported NVMe drive and exporting the drive from both the nodes,
The connection to the host fails due to the same controller ID and
results in the following error message:-

"nvme nvmeX: Duplicate cntlid XXX with nvmeX, rejecting"

With this patch now user can partition the controller IDs for each
subsystem by setting up the cntlid_min and cntlid_max. These values
will be used at the time of the controller ID creation. By partitioning
the ctrl-ids for each subsystem results in the unique ctrl-id space
which avoids the collision.

When new attribute is not specified target will fall back to original
cntlid calculation method.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>