Add a helper to avoid opencoding ns_head->ref manipulations.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NKeith Busch <kbusch@kernel.org>
Reviewed-by: NJavier González <javier.gonz@samsung.com>
Reviewed-by: NKanchan Joshi <joshi.k@samsung.com>
Reviewed-by: NChaitanya Kulkarni <chaitanya.kulkarni@wdc.com>