srv6: resolve nexthop groups in sr6_output

[rjarry]

When FRR wraps every nexthop in a group, fib6_lookup returns a group nexthop instead of the underlying L3 nexthop. In sr6_output, the secondary FIB lookup for the SID prefix is followed by a call to sr_tunsrc_get which reads nh->iface_id directly. Group nexthops have iface_id set to GR_IFACE_ID_UNDEF, causing sr_tunsrc_get to fail and dropping the packet at sr6_source_no_route.

Resolve the group to its member nexthop right after the fib6_lookup so that subsequent field accesses see the actual L3 nexthop.

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• modules/srv6/datapath/srv6_output.c

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[rjarry]

Same issue may happen in many places. I'll close this and rework it to include the nexthop group resolution inline with fib{4,6}_lookup.