rcu: Mark Hierarchical RCU no longer experimental

Removes the warnings about Hierarchical RCU being experimental,
given that it has gone through almost six months of being the
default RCU in mainline for the x86 with very little trouble.

This makes hierarchical-RCU bootup look less scary.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: akpm@linux-foundation.org
Cc: niv@us.ibm.com
Cc: dvhltc@us.ibm.com
Cc: dipankar@in.ibm.com
Cc: dhowells@redhat.com
Cc: lethal@linux-sh.org
Cc: kernel@wantstofly.org
Cc: cl@linux-foundation.org
Cc: schamp@sgi.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>