nvme: sanitize metadata bounce buffer for reads

nvme: sanitize metadata bounce buffer for reads

commit 2b32c76e2b0154b98b9322ae7546b8156cd703e6 upstream.

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: 0b7f1f26f95a51a ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>