fou: remove sparse errors

We need to add __rcu qualifier to avoid these errors:

net/ipv4/fou.c:250:18: warning: incorrect type in assignment (different address spaces)
net/ipv4/fou.c:250:18:    expected struct net_offload const **offloads
net/ipv4/fou.c:250:18:    got struct net_offload const [noderef] __rcu **
net/ipv4/fou.c:251:15: error: incompatible types in comparison expression (different address spaces):
net/ipv4/fou.c:251:15:    struct net_offload const [noderef] __rcu *
net/ipv4/fou.c:251:15:    struct net_offload const *
net/ipv4/fou.c:272:18: warning: incorrect type in assignment (different address spaces)
net/ipv4/fou.c:272:18:    expected struct net_offload const **offloads
net/ipv4/fou.c:272:18:    got struct net_offload const [noderef] __rcu **
net/ipv4/fou.c:273:15: error: incompatible types in comparison expression (different address spaces):
net/ipv4/fou.c:273:15:    struct net_offload const [noderef] __rcu *
net/ipv4/fou.c:273:15:    struct net_offload const *
net/ipv4/fou.c:442:18: warning: incorrect type in assignment (different address spaces)
net/ipv4/fou.c:442:18:    expected struct net_offload const **offloads
net/ipv4/fou.c:442:18:    got struct net_offload const [noderef] __rcu **
net/ipv4/fou.c:443:15: error: incompatible types in comparison expression (different address spaces):
net/ipv4/fou.c:443:15:    struct net_offload const [noderef] __rcu *
net/ipv4/fou.c:443:15:    struct net_offload const *
net/ipv4/fou.c:489:18: warning: incorrect type in assignment (different address spaces)
net/ipv4/fou.c:489:18:    expected struct net_offload const **offloads
net/ipv4/fou.c:489:18:    got struct net_offload const [noderef] __rcu **
net/ipv4/fou.c:490:15: error: incompatible types in comparison expression (different address spaces):
net/ipv4/fou.c:490:15:    struct net_offload const [noderef] __rcu *
net/ipv4/fou.c:490:15:    struct net_offload const *
net/ipv4/udp_offload.c:170:26: warning: incorrect type in assignment (different address spaces)
net/ipv4/udp_offload.c:170:26:    expected struct net_offload const **offloads
net/ipv4/udp_offload.c:170:26:    got struct net_offload const [noderef] __rcu **
net/ipv4/udp_offload.c:171:23: error: incompatible types in comparison expression (different address spaces):
net/ipv4/udp_offload.c:171:23:    struct net_offload const [noderef] __rcu *
net/ipv4/udp_offload.c:171:23:    struct net_offload const *

Fixes: efc98d08 ("fou: eliminate IPv4,v6 specific GRO functions")
Fixes: 8bce6d7d ("udp: Generalize skb_udp_segment")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>