sctp: add sctp_auth_init and sctp_auth_free

This patch is to factor out sctp_auth_init and sctp_auth_free
functions, and sctp_auth_init will also be used in the next
patch for SCTP_AUTH_SUPPORTED sockopt.

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