nvme: don't bother to look up a namespace for controller ioctls

Don't bother to look up a namespace just to drop if after retreiving the
controller for the multipath case.  Just look up a live controller for
the subsystem directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Javier González <javier.gonz@samsung.com>