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.

(cherry picked from commit 101a0f09e8baf8293e1eeb591de18caf15e49e00)
(cherry picked from commit 18f4b705734e1c76bac441ffe86cb8fbb131a153)

Approved by: re (cperciva)