bridge: use br_port_get_rtnl within rtnl lock

[ Upstream commit 1fb1754a8c70d69ab480763c423e0a74369c4a67 ]

current br_port_get_rcu is problematic in bridging path
(NULL deref). Change these calls in netlink path first.

Signed-off-by: Hong Zhiguo <zhiguohong@tencent.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>