Impact: When running KVM with a few VM's with physical eth bridges
and TAP interfaces connected between KVM (via libvirt) and the bridge,
occasionally kernel panics will occur.
== Fix ==
Cherry-pick from a504b86e718a425ea4a34e2f95b5cf0545ddfd8d. This is
present in Precise onwards. This fixes an issue in netfilter/tun where
tun's allocation is problematic and requires additional padding.