nvme: check ns's volatile write cache not present

When the VWC of a namespace does not exist, the BLK_FEAT_WRITE_CACHE
flag should not be set when registering the block device, regardless
of whether the controller supports VWC.

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>