if_bridge: learn IPv4 multicast listeners from IGMP

[namkhanh052005_gmail.com]

Summary

Parses IGMPv1/v2/v3 membership reports in the bridge forward path and
populates the MDB from them. Lengths and headers are validated before
parsing. IGMPv3 records are accepted only where they reduce to ASM (*,G)
interest -- at this stage, an EXCLUDE-mode record with an empty source
list; D7 widens this. Parse failures and unsupported record types create
no state.

The MDB is populated but not yet consulted; forwarding is unchanged.

Depends on D58730

Test Plan

mdb_unknown_multicast_flood ATF test confirms flood fallback is
unaffected by the new learning path. Built and booted on both bases; ATF
passes.