nvmet: add safety check for subsystem lock during nvmet_ns_changed

we need to make sure that subsystem lock is taken during ctrl's list
traversing. nvmet_ns_changed function is not static and can be used from
various callers simultaneously.

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