generalize asm-generic/ioctl.h to allow overriding values

In the spirit of a number of other asm-generic header files,
generalize asm-generic/ioctl.h to allow arch-specific ioctl.h headers
to simply override _IOC_SIZEBITS and/or _IOC_DIRBITS before including
this header file, allowing a number of ioctl.h header files to be
shortened considerably.
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/ioctl.h

@@ -21,8 +21,19 @@
  */
 #define _IOC_NRBITS	8
 #define _IOC_TYPEBITS	8
-#define _IOC_SIZEBITS	14
-#define _IOC_DIRBITS	2
+
+/*
+ * Let any architecture override either of the following before
+ * including this file.
+ */
+
+#ifndef _IOC_SIZEBITS
+# define _IOC_SIZEBITS	14
+#endif
+
+#ifndef _IOC_DIRBITS
+# define _IOC_DIRBITS	2
+#endif
 
 #define _IOC_NRMASK	((1 << _IOC_NRBITS)-1)
 #define _IOC_TYPEMASK	((1 << _IOC_TYPEBITS)-1)
@@ -35,11 +46,21 @@
 #define _IOC_DIRSHIFT	(_IOC_SIZESHIFT+_IOC_SIZEBITS)
 
 /*
- * Direction bits.
+ * Direction bits, which any architecture can choose to override
+ * before including this file.
  */
-#define _IOC_NONE	0U
-#define _IOC_WRITE	1U
-#define _IOC_READ	2U
+
+#ifndef _IOC_NONE
+# define _IOC_NONE	0U
+#endif
+
+#ifndef _IOC_WRITE
+# define _IOC_WRITE	1U
+#endif
+
+#ifndef _IOC_READ
+# define _IOC_READ	2U
+#endif
 
 #define _IOC(dir,type,nr,size) \
 	(((dir)  << _IOC_DIRSHIFT) | \