sctp: make heartbeat information in sctp_make_heartbeat()

Make heartbeat information in sctp_make_heartbeat() instead
of make it in sctp_sf_heartbeat() directly for common using.

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