linuxkpi: fix 32-bit arm build

The new KPI is only used in <asm/set_memory.h>, but it's provided in
linux_page.c.  The latter only includes the former indirectly by way of
<linux/io.h>, and that's only conditionally included outside of 32-bit
ARM there.

All of our archs have the necessary pmap_page_set_memattr(), so just
move the declaration into <linux/page.h> instead of trying to reason
about the usability of <asm/set_memory.h> directly in linux_page.c.

Reported by:	jenkins (via ivy)

sys/compat/linuxkpi/common/include/asm/set_memory.h

@@ -62,8 +62,6 @@ set_memory_wb(unsigned long addr, int numpages)
 	return (-pmap_change_attr((void *)addr, len, VM_MEMATTR_WRITE_BACK));
 }
 
-int lkpi_set_pages_attr(struct page *page, int numpages, vm_memattr_t ma);
-
 static inline int
 set_pages_uc(struct page *page, int numpages)
 {

sys/compat/linuxkpi/common/include/linux/page.h

@@ -127,4 +127,6 @@ clflush_cache_range(void *addr, unsigned int size)
 }
 #endif
 
+int lkpi_set_pages_attr(struct page *page, int numpages, vm_memattr_t ma);
+
 #endif	/* _LINUXKPI_LINUX_PAGE_H_ */