iommu/amd: Drop null test before destroy functions

Remove unneeded NULL test.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@ expression x; @@
-if (x != NULL) {
  \(kmem_cache_destroy\|mempool_destroy\|dma_pool_destroy\)(x);
  x = NULL;
-}
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd_iommu_init.c

@@ -1755,11 +1755,8 @@ static void __init free_on_init_error(void)
 	free_pages((unsigned long)irq_lookup_table,
 		   get_order(rlookup_table_size));
 
-	if (amd_iommu_irq_cache) {
-		kmem_cache_destroy(amd_iommu_irq_cache);
-		amd_iommu_irq_cache = NULL;
-
-	}
+	kmem_cache_destroy(amd_iommu_irq_cache);
+	amd_iommu_irq_cache = NULL;
 
 	free_pages((unsigned long)amd_iommu_rlookup_table,
 		   get_order(rlookup_table_size));