dccp: Upgrade NDP count from 3 to 6 bytes

RFC 4340, 7.7 specifies up to 6 bytes for the NDP Count option, whereas the code
is currently limited to up to 3 bytes. This seems to be a relict of an earlier
draft version and is brought up to date by the patch.

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