plugins/meta/sbr: Adjusted ipv6 address mask to /128
plugins/meta/sbr: Adjusted ipv6 address mask to /128

A /64 mask was used which routed an entire cidr based on source,
not only the bound address.

Fixes #478

Signed-off-by: Lars Ekman <b082ea60743bba466738fcc73ff82c52d538fed4@est.tech>

containernetworking/plugins,containernetworking/plugins