rcu/kvfree: Carefully reset number of objects in krcp

The schedule_delayed_monitor_work() function relies on the count of
objects queued into any given kfree_rcu_cpu structure.  This count is
used to determine how quickly to schedule passing these objects to RCU.

There are three pipes where pointers can be placed.  When any pipe is
offloaded, the kfree_rcu_cpu structure's ->count counter is set to zero,
which is wrong because the other pipes might still be non-empty.

This commit therefore maintains per-pipe counters, and introduces a
krc_count() helper to access the aggregate value of those counters.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>