generate_ftsn is added as a member of sctp_stream_interleave, used to
create fwdtsn or ifwdtsn chunk according to abandoned chunks, called
in sctp_retransmit and sctp_outq_sack.

sctp_generate_iftsn works for ifwdtsn, and sctp_generate_fwdtsn is
still used for making fwdtsn.
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Acked-by: NMarcelo R. Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>