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

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>