sctp: improve consistency of acc and ccc handling in snd buffer

Don't clear the counters for the socket snd buffer when
shutdown(..., SHUT_WR) or shutdown(..., SHUT_RDWR) is called.
This was causing the system to panic() when SCTP pf tests were
running.

Reported by: dchagin, kp

(cherry picked from commit efb04fb404b240a99c618e49174cd6260217edaa)