[DCCP] CCID: Allow ccid_{init,exit} to be NULL

Testing if the ccid being instantiated has these methods in
ccid_init().

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/dccp/ccid.c
Simple merge

diff --cc net/dccp/ccids/ccid3.c
Simple merge