batman-adv: Use chain pointer when purging fragments

batman-adv: Use chain pointer when purging fragments

The chain pointer was already created in batadv_frag_purge_orig to make the
checks more readable. Just use the chain pointer everywhere instead of
having the same dereference + array access in the most lines of this
function.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>