rcu: Control RCU_FANOUT_LEAF from boot-time parameter

Although making RCU_FANOUT_LEAF a kernel configuration parameter rather
than a fixed constant makes it easier for people to decrease cache-miss
overhead for large systems, it is of little help for people who must
run a single pre-built kernel binary.

This commit therefore allows the value of RCU_FANOUT_LEAF to be
increased (but not decreased!) via a boot-time parameter named
rcutree.rcu_fanout_leaf.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/kernel-parameters.txt

@@ -2367,6 +2367,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			Set maximum number of finished RCU callbacks to process
 			in one batch.
 
+	rcutree.fanout_leaf=	[KNL,BOOT]
+			Increase the number of CPUs assigned to each
+			leaf rcu_node structure.  Useful for very large
+			systems.
+
 	rcutree.qhimark=	[KNL,BOOT]
 			Set threshold of queued
 			RCU callbacks over which batch limiting is disabled.