batman-adv: make is_my_mac() check for the current mesh only

On a multi-mesh node (a node running more than one batman-adv
virtual interface) batadv_is_my_mac() has to check MAC
addresses of hard interfaces belonging to the current mesh
only.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>