rcu: Check callback-invocation time limit for rcuc kthreads

rcu: Check callback-invocation time limit for rcuc kthreads

Currently, a callback-invocation time limit is enforced only for
callbacks invoked from the softirq environment, the rationale being
that when callbacks are instead invoked from rcuc and rcuoc kthreads,
these callbacks cannot be holding up other softirq vectors.

Which is in fact true.  However, if an rcuc kthread spends too much time
invoking callbacks, it can delay quiescent-state reports from its CPU,
which can also be a problem.

This commit therefore applies the callback-invocation time limit to
callback invocation from the rcuc kthreads as well as from softirq.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>