[PATCH] Check for preemption after lowering a thread's priority

I've just put up a phabricator review for a fix to ULE and 4BSD for a
priority inversion problem.  The issue is that after a running thread's
priority is lowered (usually due to releasing a mutex and having a priority
load revoked), the scheduler does not check whether the thread should now
be preempted by a thread waiting in the runq.  See this old ML post for a
detailed description of one instance that I saw:

https://lists.freebsd.org/pipermail/freebsd-current/2013-January/039261.html

The review can be found here:
https://reviews.freebsd.org/D9518