[DCCP]: set safe upper bound for option length

This is a re-send from
http://www.mail-archive.com/dccp@vger.kernel.org/msg00553.html



It is the same patch as before, but I have built in Arnaldo's suggestions
pointed out in that posting.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>