net: Make accesses to ->br_port safe for sparse RCU

The new versions of the rcu_dereference() APIs requires that any pointers
passed to one of these APIs be fully defined.  The ->br_port field
in struct net_device points to a struct net_bridge_port, which is an
incomplete type.  This commit therefore changes ->br_port to be a void*,
and introduces a br_port() helper function to convert the type to struct
net_bridge_port, and applies this new helper function where required.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Miller <davem@davemloft.net>
Cc: Stephen Hemminger <shemminger@linux-foundation.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>