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>