ARC: mm: fix build failure in linux-next for UP builds

kisskb build service reported ARC defconfig build failures in linux-next

| arch/arc/include/asm/mmu.h:75:21: error: 'NR_CPUS' undeclared here (not in a function)
| make[3]: *** [arch/arc/mm/ioremap.o] Error 1
| make[2]: *** [arch/arc/mm] Error 2
| make[1]: *** [arch/arc] Error 2

which I bisected to a subtle side-effect of a totally benign mm patch
("mm, vmalloc: properly track vmalloc users") which caused a header
include chain deviation - asm/mmu.h using NR_CPUS before including
linux/threads.h

Fix that by adding the dependnet header and while it at fix a related
header to include linux headers aheads of asm headers as sometimes that
slso triggers such issues !

Reported-by: noreply@ellerman.id.au
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/mmu.h

@@ -9,6 +9,10 @@
 #ifndef _ASM_ARC_MMU_H
 #define _ASM_ARC_MMU_H
 
+#ifndef __ASSEMBLY__
+#include <linux/threads.h>	/* NR_CPUS */
+#endif
+
 #if defined(CONFIG_ARC_MMU_V1)
 #define CONFIG_ARC_MMU_VER 1
 #elif defined(CONFIG_ARC_MMU_V2)

arch/arc/include/asm/pgtable.h

@@ -35,11 +35,11 @@
 #ifndef _ASM_ARC_PGTABLE_H
 #define _ASM_ARC_PGTABLE_H
 
-#include <asm/page.h>
-#include <asm/mmu.h>
+#include <linux/const.h>
 #define __ARCH_USE_5LEVEL_HACK
 #include <asm-generic/pgtable-nopmd.h>
-#include <linux/const.h>
+#include <asm/page.h>
+#include <asm/mmu.h>	/* to propagate CONFIG_ARC_MMU_VER <n> */
 
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