nexthop: Convert RTM_DELNEXTHOP to per-netns RTNL.

In rtm_del_nexthop(), only nexthop_find_by_id() and remove_nexthop()
require RTNL as they touch net->nexthop.rb_root.

Let's move RTNL down as rtnl_net_lock() before nexthop_find_by_id().

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/20250319230743.65267-8-kuniyu@amazon.com


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