[DECNET] ROUTE: fix rcu_dereference() uses in /proc/net/decnet_cache

In dn_rt_cache_get_next(), no need to guard seq->private by a
rcu_dereference() since seq is private to the thread running this
function. Reading seq.private once (as guaranted bu rcu_dereference())
or several time if compiler really is dumb enough wont change the
result.
 
But we miss real spots where rcu_dereference() are needed, both in
dn_rt_cache_get_first() and dn_rt_cache_get_next()

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>