batman-adv: release vlan object after checking the CRC

There is a refcounter unbalance in the CRC checking routine
invoked on OGM reception. A vlan object is retrieved (thus
its refcounter is increased by one) but it is never properly
released. This leads to a memleak because the vlan object
will never be free'd.

Fix this by releasing the vlan object after having read the
CRC.

Reported-by: Russell Senior <russell@personaltelco.net>
Reported-by: Daniel <daniel@makrotopia.org>
Reported-by: cmsv <cmsv@wirelesspt.net>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>