batman-adv: replace WARN with rate limited output on non-existing VLAN

If a VLAN tagged frame is received and the corresponding VLAN is not
configured on the soft interface, it will splat a WARN on every packet
received. This is a quite annoying behaviour for some scenarios, e.g. if
bat0 is bridged with eth0, and there are arbitrary VLAN tagged frames
from Ethernet coming in without having any VLAN configuration on bat0.

The code should probably create vlan objects on the fly and
transparently transport these VLAN-tagged Ethernet frames, but until
this is done, at least the WARN splat should be replaced by a rate
limited output.

Fixes: 354136bc ("batman-adv: fix kernel crash due to missing NULL checks")
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>