[IPV6]: Fix potential net leak and oops in ipv6 routing code.

The commits f3db4851 ([NETNS][IPV6] ip6_fib - fib6_clean_all handle several 
network namespaces) and 69ddb805 ([NETNS][IPV6] route6 - Make proc entry 
/proc/net/rt6_stats per namespace) made some proc files per net.

Both of them introduced potential OOPS - get_proc_net can return NULL, but
this check is lost - and a struct net leak - in case single_open() fails the
previously got net is not put.

Kill all these bugs with one patch.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>