nvme: always search for namespace head

Even if a namespace reports it is not capable of sharing, search the
subsystem for a matching namespace head. If found, the driver should
reject that namespace since it's coming from an invalid configuration.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>