MIPS: pgtable-bits.h: Correct _PAGE_GLOBAL_SHIFT build failure

Correct a build failure introduced by be0c37c9 [MIPS: Rearrange PTE bits
into fixed positions.]:

In file included from ./arch/mips/include/asm/io.h:27:0,
                 from ./arch/mips/include/asm/page.h:176,
                 from include/linux/mm_types.h:15,
                 from include/linux/sched.h:27,
                 from include/linux/ptrace.h:5,
                 from arch/mips/kernel/cpu-probe.c:16:
./arch/mips/include/asm/pgtable-bits.h:164:0: error: "_PAGE_GLOBAL_SHIFT" redefined [-Werror]
 #define _PAGE_GLOBAL_SHIFT (_PAGE_MODIFIED_SHIFT + 1)
 ^
./arch/mips/include/asm/pgtable-bits.h:141:0: note: this is the location of the previous definition
 #define _PAGE_GLOBAL_SHIFT (_PAGE_SPLITTING_SHIFT + 1)
 ^
cc1: all warnings being treated as errors
make[2]: *** [arch/mips/kernel/cpu-probe.o] Error 1

for 64BIT/CPU_MIPSR1/MIPS_HUGE_TLB_SUPPORT configurations.  Remove the
scattered double `_PAGE_NO_EXEC_SHIFT' and `_PAGE_GLOBAL_SHIFT' macro
definitions and rearrange them so that the respective macros these
definitions are based on are also those used for guarding conditionals.

[ralf@linux-mips.org: resolved conflicts and updated commments.]
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9960/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/pgtable-bits.h

@@ -133,20 +133,13 @@
 #define _PAGE_HUGE		(1 << _PAGE_HUGE_SHIFT)
 #define _PAGE_SPLITTING_SHIFT	(_PAGE_HUGE_SHIFT + 1)
 #define _PAGE_SPLITTING		(1 << _PAGE_SPLITTING_SHIFT)
-
-/* Only R2 or newer cores have the XI bit */
-#if defined(CONFIG_CPU_MIPSR2) || defined(CONFIG_CPU_MIPSR6)
-#define _PAGE_NO_EXEC_SHIFT	(_PAGE_SPLITTING_SHIFT + 1)
-#else
-#define _PAGE_GLOBAL_SHIFT	(_PAGE_SPLITTING_SHIFT + 1)
-#define _PAGE_GLOBAL		(1 << _PAGE_GLOBAL_SHIFT)
-#endif	/* CONFIG_CPU_MIPSR2 || CONFIG_CPU_MIPSR6 */
-
 #endif	/* CONFIG_64BIT && CONFIG_MIPS_HUGE_TLB_SUPPORT */
 
 #if defined(CONFIG_CPU_MIPSR2) || defined(CONFIG_CPU_MIPSR6)
 /* XI - page cannot be executed */
-#ifndef _PAGE_NO_EXEC_SHIFT
+#ifdef _PAGE_SPLITTING_SHIFT
+#define _PAGE_NO_EXEC_SHIFT	(_PAGE_SPLITTING_SHIFT + 1)
+#else
 #define _PAGE_NO_EXEC_SHIFT	(_PAGE_MODIFIED_SHIFT + 1)
 #endif
 #define _PAGE_NO_EXEC		(cpu_has_rixi ? (1 << _PAGE_NO_EXEC_SHIFT) : 0)
@@ -156,14 +149,16 @@
 #define _PAGE_READ		(cpu_has_rixi ? 0 : (1 << _PAGE_READ_SHIFT))
 #define _PAGE_NO_READ_SHIFT	_PAGE_READ_SHIFT
 #define _PAGE_NO_READ		(cpu_has_rixi ? (1 << _PAGE_READ_SHIFT) : 0)
+#endif	/* defined(CONFIG_CPU_MIPSR2) || defined(CONFIG_CPU_MIPSR6) */
 
+#if defined(_PAGE_NO_READ_SHIFT)
 #define _PAGE_GLOBAL_SHIFT	(_PAGE_NO_READ_SHIFT + 1)
-#define _PAGE_GLOBAL		(1 << _PAGE_GLOBAL_SHIFT)
-
-#else	/* !CONFIG_CPU_MIPSR2 && !CONFIG_CPU_MIPSR6 */
+#elif defined(_PAGE_SPLITTING_SHIFT)
+#define _PAGE_GLOBAL_SHIFT	(_PAGE_SPLITTING_SHIFT + 1)
+#else
 #define _PAGE_GLOBAL_SHIFT	(_PAGE_MODIFIED_SHIFT + 1)
+#endif
 #define _PAGE_GLOBAL		(1 << _PAGE_GLOBAL_SHIFT)
-#endif	/* CONFIG_CPU_MIPSR2 || CONFIG_CPU_MIPSR6 */
 
 #define _PAGE_VALID_SHIFT	(_PAGE_GLOBAL_SHIFT + 1)
 #define _PAGE_VALID		(1 << _PAGE_VALID_SHIFT)