x86: separate extable.h, switch sections.h to it

drivers/platform/x86/dell-smo8800.c is touched due to the following obscenity:
drivers/platform/x86/dell-smo8800.c ->
	linux/interrupt.h ->
		linux/hardirq.h ->
			asm/hardirq.h ->
				linux/irq.h ->
					asm/hw_irq.h ->
						asm/sections.h ->
							asm/uaccess.h
is the only chain of includes pulling asm/uaccess.h there.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/drivers/platform/x86/dell-smo8800.c b/drivers/platform/x86/dell-smo8800.c
index 0aec4fd..37e6460 100644
--- a/drivers/platform/x86/dell-smo8800.c
+++ b/drivers/platform/x86/dell-smo8800.c
@@ -24,6 +24,7 @@
 #include <linux/acpi.h>
 #include <linux/interrupt.h>
 #include <linux/miscdevice.h>
+#include <linux/uaccess.h>
 
 struct smo8800_device {
 	u32 irq;                     /* acpi device irq */