dccp: Bug in initial acknowledgment number assignment

Step 8.5 in RFC 4340 says for the newly cloned socket

           Initialize S.GAR := S.ISS,

but what in fact the code (minisocks.c) does is

           Initialize S.GAR := S.ISR,

which is wrong (typo?) -- fixed by the patch.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>