SCTP: Explicitely discard OOTB chunks

Explicitely discard OOTB chunks, whether the result is a
SHUTDOWN COMPLETE or an ABORT.  We need to discard the OOTB
SHUTDOWN ACK to prevent bombing attackes since responsed
MUST NOT be bundled.  We also explicietely discard in the
ABORT case since that function is widely used internally.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>