nexthop: Convert RTM_NEWNEXTHOP to per-netns RTNL.

If we pass false to the rtnl_held param of lwtunnel_valid_encap_type(),
we can move RTNL down before rtm_to_nh_config_rtnl().

Let's use rtnl_net_lock() in rtm_new_nexthop().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250319230743.65267-7-kuniyu@amazon.com


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