x86: fix early_ioremap()/btmap

fix a long-standing weakness of the early-ioremap allocator: it
uses a single pgd entry for the boot mappings, and was not properly
protecting itself against crossing a 2MB (4MB) boundary.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/asm-x86/fixmap_32.h

@@ -90,10 +90,18 @@ enum fixed_addresses {
 	FIX_PARAVIRT_BOOTMAP,
 #endif
 	__end_of_permanent_fixed_addresses,
-	/* temporary boot-time mappings, used before ioremap() is functional */
+	/*
+	 * 256 temporary boot-time mappings, used by early_ioremap(),
+	 * before ioremap() is functional.
+	 *
+	 * We round it up to the next 512 pages boundary so that we
+	 * can have a single pgd entry and a single pte table:
+	 */
 #define NR_FIX_BTMAPS		64
 #define FIX_BTMAPS_NESTING	4
-	FIX_BTMAP_END = __end_of_permanent_fixed_addresses,
+	FIX_BTMAP_END =
+		__end_of_permanent_fixed_addresses + 512 -
+			(__end_of_permanent_fixed_addresses & 511),
 	FIX_BTMAP_BEGIN = FIX_BTMAP_END + NR_FIX_BTMAPS*FIX_BTMAPS_NESTING - 1,
 	FIX_WP_TEST,
 	__end_of_fixed_addresses