Patch from Dipankar Sarma <dipankar@in.ibm.com>

There is a check in RCU for idle CPUs which signifies quiescent state
(and hence no reference to RCU protected data) which was broken when
interrupt counters were changed to use thread_info->preempt_count.

Martin's 32 CPU machine with many idle CPUs was not completing any RCU
grace period because RCU was forever waiting for idle CPUs to context
switch.  Had the idle check worked, this would not have happened.  With
no RCU happening, the dentries were getting "freed" (dentry stats
showing that) but not getting returned to slab.  This would not show up
in systems that are generally busy as context switches then would
happen in all CPUs and the per-CPU quiescent state counter would get
incremented during context switch.