nvmet-fcloop: replace kref with refcount

The kref wrapper is not really adding any value ontop of refcount. Thus
replace the kref API with the refcount API.

Signed-off-by: Daniel Wagner <wagi@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>