rcu: Make rcu update module parameters world-readable

rcu update module parameters currently don't appear in sysfs and this is
a serviceability issue as it might be needed to access their default
values at runtime.

Fix this issue by changing rcu update module parameters permissions to
world-readable.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>