gluon-ebtables: Enable concurrent ebtables updates

This enables the ebtables internal locking mechanism which
will avoid race conditions between multiple, concurrent
ebtables calls.

This is a preparation for the upcoming gluon-arp-limiter
daemon, to avoid issues if upon restarting gluon-ebtables
the gluon-arp-limiter daemon tries to modify the tables.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>