nvme: add __exit annotation

Add __exit annotation to cleanup helper which is only
called once in the module.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/nvme.h

@@ -565,6 +565,6 @@ static inline struct nvme_ns *nvme_get_ns_from_dev(struct device *dev)
 }
 
 int __init nvme_core_init(void);
-void nvme_core_exit(void);
+void __exit nvme_core_exit(void);
 
 #endif /* _NVME_H */