ip: speedup /proc/net/rt_cache handling

When scanning route cache hash table, we can avoid taking locks for
empty buckets.  Both /proc/net/rt_cache and NETLINK RTM_GETROUTE
interface are taken into account.

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