sctp: fix auth_hmacs field's length of struct sctp_cookie

auth_hmacs field of struct sctp_cookie is used for store
Requested HMAC Algorithm Parameter, and each HMAC Identifier
is 2 bytes, so the length should be:
  SCTP_AUTH_NUM_HMACS * sizeof(__u16) + 2

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>