nvme: Use metadata for passthrough commands

The ioctls' struct allows the user to provide a metadata address and
length for a passthrough command. This patch uses these values that were
previously ignored and deletes the now unused wrapper function.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>