netns: fix net.ipv6.route.gc_min_interval_ms in netns

sysctl table was copied, all right, but ->data for net.ipv6.route.gc_min_interval_ms
was not reinitialized for "!= &init_net" case.

In init_net everthing works by accident due to correct ->data initialization
in source table.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>