Some variables used atomic_t for concurrent access while others relied on
cwnd_lock, leading to an inconsistent locking model. This can be simplified
by:
* keeping all congestion control decisions inside the cc_lock
* variables which can be accessed without a lock must use
READ_ONCE/WRITE_ONE
This is only possible, by extracting the congestion control logic from
batadv_tp_recv_ack() into a new helper batadv_tp_handle_ack(). Its
decisions are returned as a batadv_tp_ack_reaction enum value and then
applied by the caller. This separates the algorithm (deciding what to do)
from the mechanism (actually doing it).
Signed-off-by: Sven Eckelmann <sven@narfation.org>