net: fix rcu access on phonet_routes

-Add __rcu annotation on table to fix sparse warnings:
net/phonet/pn_dev.c:279:25: warning: incorrect type in assignment (different address spaces)
net/phonet/pn_dev.c:279:25:    expected struct net_device *<noident>
net/phonet/pn_dev.c:279:25:    got void [noderef] <asn:4>*<noident>
net/phonet/pn_dev.c:376:17: warning: incorrect type in assignment (different address spaces)
net/phonet/pn_dev.c:376:17:    expected struct net_device *volatile <noident>
net/phonet/pn_dev.c:376:17:    got struct net_device [noderef] <asn:4>*<noident>
net/phonet/pn_dev.c:392:17: warning: incorrect type in assignment (different address spaces)
net/phonet/pn_dev.c:392:17:    expected struct net_device *<noident>
net/phonet/pn_dev.c:392:17:    got void [noderef] <asn:4>*<noident>

-Access table with rcu_access_pointer (fixes the following sparse errors):
net/phonet/pn_dev.c:278:25: error: incompatible types in comparison expression (different address spaces)
net/phonet/pn_dev.c:391:17: error: incompatible types in comparison expression (different address spaces)

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>