pimd: send AutoRP discovery on all PIM interfaces

[enkechen-panw]

The AutoRP mapping agent was only sending discovery messages from the
single configured source interface. In topologies where routers are on
different L2 segments (not directly connected to the mapping agent's
source interface), they would not reliably receive discovery messages.

This caused flaky failures in the pim_autorp topotest, particularly
test_pim_autorp_discovery_neg_prefixes, where r4 (two hops from the
mapping agent) would intermittently fail to receive discovery.

For systems with IP_PKTINFO, fix by sending discovery on all PIM
interfaces, similar to how announcements are already sent. Use
sendmsg() with IP_PKTINFO to specify both the outgoing interface
and a consistent source address (the configured mapping agent
address). This ensures:

1. All directly connected routers receive discovery regardless of
   which interface they're on
2. The source IP is consistent across all interfaces, ensuring
   proper mapping agent election when multiple mapping agents exist

Systems without IP_PKTINFO fall back to the original single-interface
behavior.

[enkechen-panw]

ci:rerun

[mwinter-osr]

@greptileai review

[greptile-apps]

Greptile Summary

Updates AutoRP discovery distribution and coverage:

• Sends discovery packets on every eligible PIM interface when IP_PKTINFO is available while retaining the configured mapping-agent source address.
• Preserves single-interface behavior on systems without IP_PKTINFO.
• Adds a topology test covering mapping-agent election, multi-interface discovery delivery, and failback.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
pimd/pim_autorp.c	Sends AutoRP discovery through each eligible PIM interface using zero-initialized IP_PKTINFO ancillary data and retains the legacy fallback.
tests/topotests/pim_autorp/test_pim_autorp.py	Adds coverage for competing mapping agents, discovery across multiple interfaces, and restoration of the original agent.

Sequence Diagram

sequenceDiagram
    participant MA as Mapping Agent
    participant K as Kernel/IP_PKTINFO
    participant PI as PIM Interfaces
    participant R as Connected Routers
    MA->>MA: Build discovery packet
    loop Each eligible PIM interface
        MA->>K: sendmsg(packet, ifindex, configured source)
        K->>PI: Transmit on selected interface
        PI->>R: Deliver AutoRP discovery
    end
    R->>R: Elect highest-address mapping agent

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_{Reviews (2): Last reviewed commit: "tests: add AutoRP mapping agent election..." | Re-trigger Greptile}

[enkechen-panw]

@greptileai review

[enkechen-panw]

ci:rerun

[donaldsharp]

Can you help me understand why packets are not reliably being received? It's not clear to me from the description

As a note, as I read this change this code sends packets on any interface that there is a pim data structure sunk off of it, shouldn't it only send packets on the ip pim configed interfaces?