batman-adv: Set skb priority in fragments

BATMAN will set the skb->priority based on the IP precedence or 802.1q
tag. However, if it needs to fragment the frame, it currently leaves
the fragment skb with the default priority and actually overwrites the
priority in the unfragmented frame. Fix this.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>