bump flannel
bump flannel

- switch branch to new PR

Submodule src/github.com/coreos/flannel cd76b97...dfa9a3e:
  > VXLAN backend supports GBP option
  > Bump netlink to latest
  < Update README, document VXLAN GBP option
  < VXLAN backend supports GBP option
  < Switch netlink dependency to the fix-gbp branch
  < Update glide, use forked netlink remote

Signed-off-by: Gabe Rosenhouse <657db4ad0dfa95e4e10afe2fdfba83b796a49c56@pivotal.io>

cloudfoundry-incubator/cf-networking-release,cloudfoundry-incubator/cf-networking-release,cloudfoundry-incubator/cf-networking-release,cloudfoundry-incubator/cf-networking-release