net/ib: fix out-of-bounds write when active ports exceed MAX_IB_DEVS

[Peng-Xu]

Description

Fix an out-of-bounds write in the NET/IB device enumeration (ncclIbInitDevices,
src/transport/net_ib/init.cc) that crashes NCCL initialization on hosts with more
than MAX_IB_DEVS active IB/RoCE ports.

The bound ncclNIbDevs < MAX_IB_DEVS is only evaluated in the outer per-device loop
condition. The inner per-port loop and the data-direct sub-loop both write to
ncclIbDevs[ncclNIbDevs] and increment ncclNIbDevs without re-checking the bound,
so a single multi-port device can push ncclNIbDevs past MAX_IB_DEVS within one
outer iteration and write past the end of the global ncclIbDevs[MAX_IB_DEVS] array.
The overflowed index is also handed to the per-device async thread and later to
qsort().

This guards both inner loops with the same ncclNIbDevs < MAX_IB_DEVS check so
enumeration stops cleanly at the limit, and adds a one-time WARN when the limit is
reached so the ignored ports are not silent. MAX_IB_DEVS itself is intentionally
left unchanged -- this PR only prevents the overflow; whether the limit should be
raised/made dynamic is a separate discussion.

Related Issues

Fixes #2282

Changes & Impact

• src/transport/net_ib/init.cc:
  • add && ncclNIbDevs < MAX_IB_DEVS to the per-port loop and the data-direct
    (dev < devCount) loop conditions;
  • add a single WARN after enumeration when ncclNIbDevs >= MAX_IB_DEVS,
    pointing users to NCCL_IB_HCA for device selection.
• No public API / ABI changes.
• Behavior change only on nodes that exceed MAX_IB_DEVS active ports: previously
  undefined behavior / crash, now capped at MAX_IB_DEVS with a warning. Nodes with
  <= MAX_IB_DEVS active ports are byte-for-byte unaffected (the guard never fires).

Performance Impact

None. The added checks are integer comparisons on the cold initialization path;
the data path is untouched.

Testing

• Builds cleanly (0 warnings, 0 errors) with CUDA 13.2 and NCCL's default flags.
• Reproduced on a host with 22 IB devices / 54 active RoCE ports:
  • Before (default MAX_IB_DEVS=32, unpatched): NCCL crashes during
    ncclIbInitDevices (out-of-bounds write on ncclIbDevs[32]).
  • After (patched, MAX_IB_DEVS=32): all_reduce_perf -g 8 initializes and
    completes correctly (# Out of bounds values : 0 OK), and NCCL prints exactly
    one warning from net_ib/init.cc:
    NET/IB : Reached MAX_IB_DEVS=32 IB devices; any additional ports are not used.