[NETNS][IPV6] route6 - Make ip6_dst_gc simpler

This patches improves the readibility of the ip6_dst_gc() routine.
It simplifies long lines which grow a lot due to the introduction
of network namespaces support.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>