nvme: fix handling of metadata_len for NVME_IOCTL_IO_CMD

The old code in nvme_user_cmd() passed the userspace virtual address
from nvme_passthru_cmd.metadata as the length of the metadata buffer
as well as the address to nvme_submit_user_cmd().

Fixes: 63263d60 ("nvme: Use metadata for passthrough commands")
Signed-off-by: Roland Dreier <roland@purestorage.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>