RISC-V: Fix Maximum Physical Memory 2GiB option for 64bit systems

The Maximum Physical Memory 2GiB option for 64bit systems is currently
broken because kernel hangs at boot-time when this option is enabled
and the underlying system has more than 2GiB memory.

This issue can be easily reproduced on SiFive Unleashed board where
we have 8GiB of memory.

This patch fixes above issue by removing unusable memory region in
setup_bootmem().
Signed-off-by: NAnup Patel <anup.patel@wdc.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

arch/riscv/mm/init.c

@@ -121,6 +121,14 @@ void __init setup_bootmem(void)
 			 */
 			memblock_reserve(reg->base, vmlinux_end - reg->base);
 			mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
+
+			/*
+			 * Remove memblock from the end of usable area to the
+			 * end of region
+			 */
+			if (reg->base + mem_size < end)
+				memblock_remove(reg->base + mem_size,
+						end - reg->base - mem_size);
 		}
 	}
 	BUG_ON(mem_size == 0);