nvmet: add metadata support for block devices

Allocate the metadata SGL buffers and set metadata fields for the
request. Then create a block IO request for the metadata from the
protection SG list.

Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>