powerpc/kernel: Avoid preemption check in iommu_range_alloc()

Replace the __this_cpu_read() with raw_cpu_read() in
iommu_range_alloc(). Otherwise we get a warning about using
__this_cpu_read() in preemptible code:

  BUG: using __this_cpu_read() in preemptible
  caller is iommu_range_alloc+0xa8/0x3d0

Preemption doesn't need to be disabled since according to the comment
any CPU can safely use any IOMMU pool.
Signed-off-by: Victor Aoqui <victora@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/iommu.c

@@ -208,7 +208,7 @@ static unsigned long iommu_range_alloc(struct device *dev,
 	 * We don't need to disable preemption here because any CPU can
 	 * safely use any IOMMU pool.
 	 */
-	pool_nr = __this_cpu_read(iommu_pool_hash) & (tbl->nr_pools - 1);
+	pool_nr = raw_cpu_read(iommu_pool_hash) & (tbl->nr_pools - 1);
 
 	if (largealloc)
 		pool = &(tbl->large_pool);