nvme: sanitize metadata bounce buffer for reads

User can request more metadata bytes than the device will write. Ensure
kernel buffer is initialized so we're not leaking unsanitized memory on
the copy-out.

Fixes: 0b7f1f26

 ("nvme: use the block layer for userspace passthrough metadata")
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>