mpls: no longer hold RTNL in mpls_netconf_dump_devconf()

mpls: no longer hold RTNL in mpls_netconf_dump_devconf()

- Use for_each_netdev_dump() to no longer rely
  on net->dev_index_head hash table.

- No longer care of net->dev_base_seq

- Fix return value at the end of a dump,
  so that NLMSG_DONE can be appended to current skb,
  saving one recvmsg() system call.

- No longer grab RTNL, RCU protection is enough,
  afer adding one READ_ONCE(mdev->input_enabled)
  in mpls_netconf_fill_devconf()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240410111951.2673193-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>