RDMA: fix cross-NIC same-host IPv6 RDMA-CM connect

RDMA-CM cannot establish an IPv6 RoCEv2 connection between two NICs that live on the same host. This shows up on hosts that pin one process per NUMA-local NIC and let those processes talk to each other over each NIC's global IPv6 GID (e.g. a storage daemon with one engine per NUMA node on dual ConnectX-7). rdma_resolve_addr() and ib_send_cm_req() both return success, but the destination NIC silently drops the frame and the peer never sees the REQ; the connection times out.

The bug has two halves, one on each side of the connection:

1) Send side (patch 1, drivers/infiniband/core/addr.c)

When the destination address is local, addr_resolve_neigh() copies the *source* device's MAC into the path record's destination MAC. That is right for true loopback (same netdev), but for a destination that lives on a different netdev of the same host the destination NIC will not accept a frame addressed to the source NIC's MAC and drops it in HW. The fix resolves the netdev that owns the destination address and uses its MAC.

2) Receive side (patch 2, drivers/infiniband/core/cma.c)

Once the REQ does reach the peer, validate_ipv6_net_dev() rejects it:
rt6_lookup() of a same-host destination collapses onto the loopback netdev, so the strict rt6i_idev->dev == net_dev check fails with -EHOSTUNREACH even though the REQ arrived on the right net_dev. The fix accepts an RTF_LOCAL route when net_dev itself owns the listener address. This half is only observable once patch 1 lets the REQ arrive.

Both halves are needed for a working connection; patch 1 alone makes the REQ reach the peer but it is then rejected by the unfixed receive side.

Verification
------------
Measured on two RoCEv2 ConnectX-7 ports on the same host, each with a global IPv6 GID (port A "src", port B "dst"), driving a cross-NIC RDMA-CM connect (rping, src GID on port A -> dst GID on port B) while tracing the destination MAC resolved in addr_resolve():

without the series: resolved dst MAC = port A's MAC (the *source* NIC)

                        -> frame dropped, connect times out
  with the series:     resolved dst MAC = port B's MAC (the *dest* NIC)
                        -> connect completes

The kernel under test carried c31e4038c97f and its dst_rtable() prereq (i.e. the same addr_resolve_neigh()/is_dst_local() shape as for-next); the change applies unmodified to rdma.git for-next.

Note on stable: the Fixes: tags bound the backport to where each construct exists in its current form. Trees predating c31e4038c97f have the equivalent send-side gap in the older IFF_LOOPBACK form of addr_resolve_neigh() and would need a separately shaped backport.

The patches are independent files but should be applied as a pair so the connection works end to end.

Alex Timofeyev (2):
RDMA/core: use destination netdev MAC for cross-NIC same-host local
dst
RDMA/cma: accept cross-NIC same-host local dst in
validate_ipv6_net_dev

 drivers/infiniband/core/addr.c | 22 +++++++++++++++++++---
 drivers/infiniband/core/cma.c  | 15 ++++++++++++++-
 2 files changed, 33 insertions(+), 4 deletions(-)

base-commit: 20ff9350862468af21b46cae2c22d17d6ec637f9

2.40.4