CVE-2026-68201: ALSA: timer: drain a slave's callback before its master detaches it

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

ALSA: timer: drain a slave's callback before its master detaches it

sndtimercloselocked() drains the closing instance's own in-flight callback (IFLGCALLBACK) before freeing it, but not its slaves'. When a master instance is closed, removeslavelinks() clears each slave's ->timer; the slave's own close then reads timer == NULL and takes the branch that skips the drain entirely (sndtimerstopslave() also no-ops on a NULL timer). So a slave whose callback is still running when the master is closed is freed underneath the live callback, leading to use-after-free.

Drain the slaves too before removeslavelinks() severs them. sndtimerstop() has already taken this instance off the active list, so no new slave callback can be queued. Take the slaves off the ack list so a pending one can't fire either, then wait for any that is already in flight.