batman-adv: tp_meter: consolidate congestion control variables

batman-adv: tp_meter: consolidate congestion control variables

Congestion control variables in batadv_tp_sender were scattered across the
struct without clear grouping, making it difficult to reason about which
fields require cwnd_lock (now "cc_lock") protection. These should be
combined in a structure to make it more easily visible which variable
should be read/modified with the cc_lock held.

Signed-off-by: Sven Eckelmann <sven@narfation.org>