This function has a loop where it attempts to lock all channels in a
group. If doing so would block, it releases all locks, sleeps for a
bit, and tries again. However, once the syncgroup lock is dropped,
nothing prevents the syncgroup structure from being freed.

This bug can be exploited to get escalate privileges. Reliable exploitation
likely requires access to more than one PCM device.

Fix the inner loop: after waking up, break out of it unconditionally and
start everything again. I think the old code was also buggy and not
well-exercised: after waking up we'd continue to try and continue
locking channels. Then we'd try again from the beginning and fail to
lock the channels we had already locked.

Reported by: Hazley Samsudin of GovTech CSG