enabling kvm w/SPLIT_PTLOCK_CPUS > NR_CPUS fails to build in pgtable.c

[Mike Frysinger <vapier@xxxxxxxxxx>]

i've configured my kernel to have:
	CONFIG_KVM=y
	CONFIG_NR_CPUS=2
	CONFIG_SPLIT_PTLOCK_CPUS=4

and that makes the build very unhappy:
	  CC      arch/s390/mm/pgtable.o
	arch/s390/mm/pgtable.c: In function ‘__gmap_segment_gaddr’:
	arch/s390/mm/pgtable.c:327:2: error: implicit declaration of function 
	‘pmd_to_page’ [-Werror=implicit-function-declaration]
	  page = pmd_to_page((pmd_t *) entry);
	  ^

this is because include/linux/mm_types.h has:
	#define USE_SPLIT_PMD_PTLOCKS	(USE_SPLIT_PTE_PTLOCKS && \
			IS_ENABLED(CONFIG_ARCH_ENABLE_SPLIT_PMD_PTLOCK))

and include/linux/mm.h has:
	#if USE_SPLIT_PMD_PTLOCKS
	static struct page *pmd_to_page(pmd_t *pmd)
	...

i did set NR_CPUS down on purpose (this system only has two cpus), but 
apparently tweaking SPLIT_PTLOCK_CPUS isn't an option :).  in mm/Kconfig:
	config SPLIT_PTLOCK_CPUS
		int
		default "4"
-mike

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