sctp: improve input validation for data chunks

fsn_included should only be considered, if first_frag_seen is true.
Also, fix the resetting of the control structure, if stream queues
are flushed.
This fixes a bug where a legitimate message sequence was incorrectly
classified as illegitimate.
Thanks to Victor Boivie for reporting the issue on the userland
stack.

MFC after: 3 days