nexthop: Check NLM_F_REPLACE and NHA_ID in rtm_new_nexthop().

nexthop_add() checks if NLM_F_REPLACE is specified without
non-zero NHA_ID, which does not require RTNL.

Let's move the check to 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/20250319230743.65267-5-kuniyu@amazon.com


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