x86: Fix ASM_X86__ header guards

Change header guards named "ASM_X86__*" to "_ASM_X86_*" since:

a. the double underscore is ugly and pointless.
b. no leading underscore violates namespace constraints.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/arch/x86/include/asm/pgtable-2level-defs.h b/arch/x86/include/asm/pgtable-2level-defs.h
index 7ec48f4..d77db89 100644
--- a/arch/x86/include/asm/pgtable-2level-defs.h
+++ b/arch/x86/include/asm/pgtable-2level-defs.h
@@ -1,5 +1,5 @@
-#ifndef ASM_X86__PGTABLE_2LEVEL_DEFS_H
-#define ASM_X86__PGTABLE_2LEVEL_DEFS_H
+#ifndef _ASM_X86_PGTABLE_2LEVEL_DEFS_H
+#define _ASM_X86_PGTABLE_2LEVEL_DEFS_H
 
 #define SHARED_KERNEL_PMD	0
 
@@ -17,4 +17,4 @@
 
 #define PTRS_PER_PTE	1024
 
-#endif /* ASM_X86__PGTABLE_2LEVEL_DEFS_H */
+#endif /* _ASM_X86_PGTABLE_2LEVEL_DEFS_H */