nvme: change locking for the per-subsystem controller list

Life becomes a lot simpler if we just use the global
nvme_subsystems_lock to protect this list.  Given that it is only
accessed during controller probing and removal that isn't a scalability
problem either.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>