[DCCP]: Sequence number wrap-around when sending reset

This replaces normal addition with mod-48 addition so that sequence number
wraparound is respected.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>