rcu: use rcu softirq for forcing quiescent state

rcu: use rcu softirq for forcing quiescent state

As rcu callbacks are processed in __do_softirq() there is no need to
use the scheduling softirq for forcing quiescent state. Any other
softirq would do the job and the scheduling one is the most expensive.

So use the already existing rcu softirq for that purpose. For telling
apart why the rcu softirq was raised add a flag for the current usage.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>