Staging: batman-adv: Fix skbuff leak in VIS code.

The vis code takes a copy of the data inside the skbuf if it is interesting
for us, so we always need to release the skbuf.

Reported-by: Linus Luessing <linus.luessing@web.de>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>