sched_ule: Ensure we hold the thread lock when modifying td_flags

[markj]

Summary

The load balancer may force a running thread to reschedule and pick a
new CPU. To do this it sets some flags in the thread running on a
loaded CPU. But it assumes that the thread's lock is equal to that of
the runqueue, and there are small windows where this is not true. In
this case, we can end up with non-atomic modifications to td_flags.

Since this load balancing is best-effort, simply give up if the thread's
lock doesn't match; in this case the thread is about to enter the
scheduler anyway.

Reported by: glebius
Fixes: e745d729be60 ("sched_ule(4): Improve long-term load balancer.")