nvmet: delete controllers deletion upon subsystem release

No reason for them to be kept around if we are
deleting the subsystem, so instead of passively
wait for the host to disconnect, actively delete
the controllers.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>