dccp: generalise data-loss condition

This patch generalises the task of determining data loss from RFC 4340, 7.7.1.

Let S_A, S_B be sequence numbers such that S_B is "after" S_A, and let
N_B be the NDP count of packet S_B. Then, using modulo-2^48 arithmetic,
 D = S_B - S_A - 1  is an upper bound of the number of lost data packets,
 D - N_B            is an approximation of the number of lost data packets
                    (there are cases where this is not exact).

The patch implements this as
 dccp_loss_count(S_A, S_B, N_B) := max(S_B - S_A - 1 - N_B, 0)

Signed-off-by: Ivo Calado <ivocalado@embedded.ufcg.edu.br>
Signed-off-by: Erivaldo Xavier <desadoc@gmail.com>
Signed-off-by: Leandro Sales <leandroal@gmail.com>
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>