sctp: Fix mis-ordering of user space data when multihoming in use

sctp: Fix mis-ordering of user space data when multihoming in use

Recently had a bug reported to me, in which the user was sending
packets with a payload containing a sequence number.  The packets
were getting delivered in order according the chunk TSN values, but
the sequence values in the payload were arriving out of order.  At
first I thought it must be an application error, but we eventually
found it to be a problem on the transmit side in the sctp stack.

The conditions for the error are that multihoming must be in use,
and it helps if each transport has a different pmtu.  The problem
occurs in sctp_outq_flush.  Basically we dequeue packets from the
data queue, and attempt to append them to the orrered packet for a
given transport.  After we append a data chunk we add the trasport
to the end of a list of transports to have their packets sent at
the end of sctp_outq_flush.  The problem occurs when a data chunks
fills up a offered packet on a transport.  The function that does
the appending (sctp_packet_transmit_chunk), will try to call
sctp_packet_transmit on the full packet, and then append the chunk
to a new packet.  This call to sctp_packet_transmit, sends that
packet ahead of the others that may be queued in the transport_list
in sctp_outq_flush.  The result is that frames that were sent in one
order from the user space sending application get re-ordered prior
to tsn assignment in sctp_packet_transmit, resulting in mis-sequencing
of data payloads, even though tsn ordering is correct.

The fix is to change where we assign a tsn.  By doing this earlier,
we are then free to place chunks in packets, whatever way we
see fit and the protocol will make sure to do all the appropriate
re-ordering on receive as is needed.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: William Reich <reich@ulticom.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>