riscv: Fixup pfn_valid error with wrong max_mapnr

The max_mapnr is the number of PFNs, not absolute PFN offset.
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Fixes: d0d8aae6 ("RISC-V: Set maximum number of mapped pages correctly")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/mm/init.c

@@ -196,7 +196,7 @@ void __init setup_bootmem(void)
 	max_pfn = PFN_DOWN(dram_end);
 	max_low_pfn = max_pfn;
 	dma32_phys_limit = min(4UL * SZ_1G, (unsigned long)PFN_PHYS(max_low_pfn));
-	set_max_mapnr(max_low_pfn);
+	set_max_mapnr(max_low_pfn - ARCH_PFN_OFFSET);
 
 #ifdef CONFIG_BLK_DEV_INITRD
 	setup_initrd();