SCTP: Assign stream sequence numbers to the entire message

Currently we only assign the sequence number to a packet that
we are about to transmit.  This however breaks the Partial
Reliability extensions, because it's possible for us to
never transmit a packet, i.e. it expires before we get to send
it.  In such cases, if the message contained multiple SCTP
fragments, and we did manage to send the first part of the
message, the Stream sequence numbers would get into invalid
state and cause receiver to stall.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>