rtnetlink: remove pad field in ndo_fdb_dump_context

I chose to remove this field in a separate patch to ease
potential bisection, in case one ndo_fdb_dump() is still
using the old way (cb->args[2] instead of ctx->fdb_idx)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241209100747.2269613-4-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>