MFS r361469: Fix bug in PR-SCTP
Only drop DATA chunk with lower priorities as specified in RFC 7496.
This issue was found by looking at a reproducer generated by syzkaller.

MFS r361472: Improve SCTP iterator
Ensure that the SCTP iterator runs with an stcb and inp, which belong to
each other.

MFS r361473: Improve stcb handling during teardown
Ensure that an stcb is not dereferenced when it is about to be freed.
This issue was found by SYZKALLER.

MFS r361476: Improve ASCONF handling
Avoid an integer underflow.

Approved by: re(gjb)