mctp: no longer rely on net->dev_index_head[]

mctp: no longer rely on net->dev_index_head[]

mctp_dump_addrinfo() is one of the last users of
net->dev_index_head[] in the control path.

Switch to for_each_netdev_dump() for better scalability.

Use C99 for mctp_device_rtnl_msg_handlers[] to prepare
future RTNL removal from mctp_dump_addrinfo()

(mdev->addrs is not yet RCU protected)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Matt Johnston <matt@codeconstruct.com.au>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://patch.msgid.link/20241206223811.1343076-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>