macvlan: silence RCU list debugging warning

macvlan_hash_lookup() uses list_for_each_entry_rcu() for traversing
should either under RCU in fast path or the protection of rtnl_mutex.

In the case of holding RTNL, we should add the corresponding lockdep
expression to silence the following false-positive warning:

=============================
WARNING: suspicious RCU usage
5.7.0-rc1-next-20200416-00003-ga3b8d28bc #1 Not tainted
-----------------------------
drivers/net/macvlan.c:126 RCU-list traversed in non-reader section!!

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>