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/dma-mapping.h b/arch/x86/include/asm/dma-mapping.h
index 219c33d..4a5397b 100644
--- a/arch/x86/include/asm/dma-mapping.h
+++ b/arch/x86/include/asm/dma-mapping.h
@@ -1,5 +1,5 @@
-#ifndef ASM_X86__DMA_MAPPING_H
-#define ASM_X86__DMA_MAPPING_H
+#ifndef _ASM_X86_DMA_MAPPING_H
+#define _ASM_X86_DMA_MAPPING_H
 
 /*
  * IOMMU interface. See Documentation/DMA-mapping.txt and DMA-API.txt for
@@ -65,7 +65,7 @@
 		return dma_ops;
 	else
 		return dev->archdata.dma_ops;
-#endif /* ASM_X86__DMA_MAPPING_H */
+#endif /* _ASM_X86_DMA_MAPPING_H */
 }
 
 /* Make sure we keep the same behaviour */