powerpc: Use is_kdump_kernel()

linux/crash_dump.h defines is_kdump_kernel() to be used by code that
needs to know if the previous kernel crashed instead of a (clean) boot
or reboot.

This updates the just added powerpc code to use it.  This is needed
for the next commit, which will remove __kdump_flag.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
index 3857d7e..45f47c9 100644
--- a/arch/powerpc/kernel/iommu.c
+++ b/arch/powerpc/kernel/iommu.c
@@ -32,6 +32,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/bitops.h>
 #include <linux/iommu-helper.h>
+#include <linux/crash_dump.h>
 #include <asm/io.h>
 #include <asm/prom.h>
 #include <asm/iommu.h>
@@ -460,7 +461,7 @@
 
 static void iommu_table_clear(struct iommu_table *tbl)
 {
-	if (!__kdump_flag) {
+	if (!is_kdump_kernel()) {
 		/* Clear the table in case firmware left allocations in it */
 		ppc_md.tce_free(tbl, tbl->it_offset, tbl->it_size);
 		return;