IB/core: Fix zero dmac race in neighbor resolution

dst_fetch_ha() checks nud_state without holding the neighbor lock, then
copies ha under the seqlock. A race in __neigh_update() where nud_state
is set to NUD_REACHABLE before ha is written allows dst_fetch_ha() to
read a zero MAC address while the seqlock reports no concurrent writer.

netevent_callback amplifies this by waking ALL pending addr_req workers
when ANY neighbor becomes NUD_VALID. At scale (N peers resolving ARP
concurrently), the hit probability scales as N^2, making it near-certain
for large RDMA workloads.

N(A): neigh_update(A) W(A): addr_resolve(A)
| [sleep]
| write_lock_bh(&A->lock) |
| A->nud_state = NUD_REACHABLE |
| // A->ha is still 0 |
| [woken by netevent_cb() of
| another neighbour]
| | dst_fetch_ha(A)
| | A->nud_state & NUD_VALID
| | read_seqbegin(&A->ha_lock)
| | snapshot = A->ha /* 0 */
| | read_seqretry(&A->ha_lock)
| | return snapshot
| seqlock(&A->ha_lock)
| A->ha = mac_A /* too late */
| sequnlock(&A->ha_lock)
| write_unlock_bh(&A->lock)

The incorrect/zero mac is read and programmed in the device QP while it
was not yet updated. This causes silent packet loss and eventual
RETRY_EXC_ERR.

Fix by holding the neighbor read lock across the nud_state check and
ha copy in dst_fetch_ha(), ensuring it synchronizes with
__neigh_update() which is updating while holding the write lock.

Cc: stable@vger.kernel.org
Fixes: 92ebb6a0a13a ("IB/cm: Remove now useless rcu_lock in dst_fetch_ha")
Link: https://patch.msgid.link/r/20260405-fix-dmac-race-v1-1-cfa1ec2ce54a@nvidia.com
Signed-off-by: Chen Zhao <chezhao@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>