Additive to the existing TCP listener — clients choose one transport
per mount, so adding RDMA doesn't disrupt anything. The hardware path
exists: mlx5_bond_0 (the LACP bond's RDMA representation) is ACTIVE
with link_layer=Ethernet (RoCEv2). Bonded RoCE on ConnectX-5 surfaces
both 25 GbE slaves as a single RDMA device, so RDMA traffic uses the
full 50 Gbps aggregate via the firmware's own LAG handling.

Clients (door-pek) use proto=rdma,port=20049 in nfs.nix to opt in.
RDMA transports have intrinsic parallelism (queue pairs), so nconnect
becomes a no-op — drop it from the mount options when switching.

Idempotent listener registration: nfsd's portlist accepts duplicate
adds with EINVAL, so the oneshot pre-checks before writing.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>