If a broadcast packet is coming from a client marked as
isolated, then mark the skb using the isolation mark so
that netfilter (or any other application) can recognise
them.

The mark is written in the skb based on the mask value:
only bits set in the mask are substitued by those in the
mark value
Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>