rtnetlink: Allocate linkinfo[] as struct rtnl_newlink_tbs.

We will move linkinfo to rtnl_newlink() and pass it down to other
functions.

Let's pack it into rtnl_newlink_tbs.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>