rcu/kvfree: Do not run a page work if a cache is disabled

rcu/kvfree: Do not run a page work if a cache is disabled

By default the cache size is 5 pages per CPU, but it can be disabled at
boot time by setting the rcu_min_cached_objs to zero.  When that happens,
the current code will uselessly set an hrtimer to schedule refilling this
cache with zero pages.  This commit therefore streamlines this process
by simply refusing the set the hrtimer when rcu_min_cached_objs is zero.

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