rcu-tasks: Make ksoftirqd provide RCU Tasks quiescent states

Heavy networking load can cause a CPU to execute continuously and
indefinitely within ksoftirqd, in which case there will be no voluntary
task switches and thus no RCU-tasks quiescent states.  This commit
therefore causes the exiting rcu_softirq_qs() to provide an RCU-tasks
quiescent state.

This of course means that __do_softirq() and its callers cannot be
invoked from within a tracing trampoline.

Reported-by: Toke Høiland-Jørgensen <toke@redhat.com>
Tested-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>