sctp: delete the nested flexible array hmac

This patch deletes the flexible-array hmac[] from the structure
sctp_authhdr to avoid some sparse warnings:

  # make C=2 CF="-Wflexible-array-nested" M=./net/sctp/
  net/sctp/auth.c: note: in included file (through include/net/sctp/structs.h, include/net/sctp/sctp.h):
  ./include/linux/sctp.h:735:29: warning: nested flexible array

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