nexthop: Split nh_check_attr_group().

nexthop: Split nh_check_attr_group().

We will push RTNL down to rtm_new_nexthop(), and then we
want to move non-RTNL operations out of the scope.

nh_check_attr_group() validates NHA_GROUP attributes, and
nexthop_find_by_id() and some validation requires RTNL.

Let's factorise such parts as nh_check_attr_group_rtnl()
and call it from rtm_to_nh_config_rtnl().

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/20250319230743.65267-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>