rcu: don't process callbacks when holding a rcu_read_lock()

rcu: don't process callbacks when holding a rcu_read_lock()

Some keyhandlers are calling process_pending_softirqs() while holding
a rcu_read_lock(). This is wrong, as process_pending_softirqs() might
activate rcu calls which should not happen inside a rcu_read_lock().

For that purpose modify process_pending_softirqs() to not allow rcu
callback processing when a rcu_read_lock() is being held.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>