sched_ule: Ensure we hold the thread lock when modifying td_flags

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 the code assumed that a running thread's lock is the
same as that of the corresponding 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.

Reviewed by: kib
Reported by: glebius
Fixes: e745d729be60 ("sched_ule(4): Improve long-term load balancer.")
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35821