nvme-rdma: add metadata/T10-PI support

For capable HCAs (e.g. ConnectX-5/ConnectX-6) this will allow end-to-end
protection information passthrough and validation for NVMe over RDMA
transport. Metadata offload support was implemented over the new RDMA
signature verbs API and it is enabled for capable controllers.

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