ia64: simplify code flow around swiotlb init

Before the change CONFIG_INTEL_IOMMU && !CONFIG_SWIOTLB && !CONFIG_FLATMEM
could skip `set_max_mapnr(max_low_pfn);` if iommu is not present on
system.

Link: https://lkml.kernel.org/r/20210328202439.403601-1-slyfox@gentoo.org
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/ia64/mm/init.c

@@ -644,13 +644,16 @@ mem_init (void)
 	 * _before_ any drivers that may need the PCI DMA interface are
 	 * initialized or bootmem has been freed.
 	 */
+	do {
 #ifdef CONFIG_INTEL_IOMMU
-	detect_intel_iommu();
-	if (!iommu_detected)
+		detect_intel_iommu();
+		if (iommu_detected)
+			break;
 #endif
 #ifdef CONFIG_SWIOTLB
 		swiotlb_init(1);
 #endif
+	} while (0);
 
 #ifdef CONFIG_FLATMEM
 	BUG_ON(!mem_map);