handle_ftsn is added as a member of sctp_stream_interleave, used to skip
ssn for data or mid for idata, called for SCTP_CMD_PROCESS_FWDTSN cmd.

sctp_handle_iftsn works for ifwdtsn, and sctp_handle_fwdtsn works for
fwdtsn. Note that different from sctp_handle_fwdtsn, sctp_handle_iftsn
could do stream abort pd.
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>