nexthop: Move nlmsg_parse() in rtm_to_nh_config() to rtm_new_nexthop().

We will split rtm_to_nh_config() into non-RTNL and RTNL parts,
and then the latter also needs tb.

As a prep, let's move nlmsg_parse() to rtm_new_nexthop().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250319230743.65267-2-kuniyu@amazon.com


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