fib_hash: fix rcu sparse and logical errors

While fixing CONFIG_SPARSE_RCU_POINTER errors, I had to fix accesses to
fz->fz_hash for real.

-	&fz->fz_hash[fn_hash(f->fn_key, fz)]
+	rcu_dereference(fz->fz_hash) + fn_hash(f->fn_key, fz)

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>