sctp: use ep and asoc auth_enable properly

sctp has per endpoint auth flag and per asoc auth flag, and
the asoc one should be checked when coming to asoc and the
endpoint one should be checked when coming to endpoint.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>