MIPS: mm: Fix highmem compile

Commit a5718fe8 ("MIPS: mm: Drop boot_mem_map") removed the
definition of a page variable for some reason, but that variable is
still used. Restore it to fix compilation with CONFIG_HIGHMEM enabled.
Signed-off-by: Paul Burton <paul.burton@mips.com>

arch/mips/mm/init.c

@@ -434,6 +434,8 @@ static inline void __init mem_init_free_highmem(void)
 		return;
 
 	for (tmp = highstart_pfn; tmp < highend_pfn; tmp++) {
+		struct page *page = pfn_to_page(tmp);
+
 		if (!memblock_is_memory(PFN_PHYS(tmp)))
 			SetPageReserved(page);
 		else