powerpc/perf/hv-24x7: Add missing put_cpu_var()

Add missing put_cpu_var() for 24x7 requests. This went missing in
commit f34b6c72 (3.18-rc3).

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>