CVE-2026-52929: sctp: stream: fully roll back denied add-stream state

In the Linux kernel, the following vulnerability has been resolved:

sctp: stream: fully roll back denied add-stream state

When ADDOUTSTREAMS is denied, SCTP only shrinks the queued chunks and then lowers outcnt. That leaves removed stream metadata behind, so a later re-add can reuse a stale ext and hit a null-pointer dereference in the scheduler get path.

Fix the rollback by tearing down the removed stream state the same way other stream resizes do. Unschedule the current scheduler state, drop the removed stream ext state with sctpstreamoutqmigrate(), and then reschedule the remaining streams.

This keeps scheduler-private RR/FC/PRIO lists consistent while fully rolling back denied outgoing stream additions.