NFSv4/pNFS: Store the transport type in struct nfs4_pnfs_ds_addr

We want to enable RDMA and UDP as valid transport methods if a
GETDEVICEINFO call specifies it. Do so by adding a parser for the
netid that translates it to an appropriate argument for the RPC
transport layer.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>