nvmet: configfs: use ctrl->instance to track passthru subsystems

nvmet: configfs: use ctrl->instance to track passthru subsystems

To prevent enabling more than one passthrough subsystem per NVMe
controller, passthru.c maintains an xarray indexed by cntlid values.
Passthrough for a given nvmet subsystem cannot be enabled by configfs
if the subsystem's passthru_ctrl->cntlid value is already accounted
for in the xarray.

However, according to the NVMe spec (rev 2.0c, p.145), "The Controller
ID (CNTLID) value returned in the Identify Controller data structure
may be used to uniquely identify a controller within an NVM subsystem,"
meaning that cntlid values are not guaranteed to be globally unique
across multiple subsystems. Instead, the cntlid only uniquely
identifies multiple controllers _within_ a subsystem.

As a result, multiple unique & valid NVMe targets can be blocked from
enabling passthrough at the same time if their controllers share cntlid
values, a behavior allowed by the spec. Fix this by indexing the xarray
with passthru_ctrl->instance values, which are allocated per
controller by IDA and thus should be truly unique.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Evan Burgess <evan.burgess@seagate.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>