netns: no longer hold RTNL in rtnl_net_dumpid()

- rtnl_net_dumpid() is already fully RCU protected,
  RTNL is not needed there.

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

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20240416140739.967941-1-edumazet@google.com


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