RISC-V: Fix usage of memblock_enforce_memory_limit

memblock_enforce_memory_limit accepts the maximum memory size not the
maximum address that can be handled by kernel. Fix the function invocation
accordingly.

Fixes: 1bd14a66 ("RISC-V: Remove any memblock representing unusable memory area")
Cc: stable@vger.kernel.org
Reported-by: NBin Meng <bin.meng@windriver.com>
Tested-by: NBin Meng <bin.meng@windriver.com>
Acked-by: NMike Rapoport <rppt@linux.ibm.com>
Signed-off-by: NAtish Patra <atish.patra@wdc.com>
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/mm/init.c

@@ -176,7 +176,7 @@ void __init setup_bootmem(void)
 	 * Make sure that any memory beyond mem_start + (-PAGE_OFFSET) is removed
 	 * as it is unusable by kernel.
 	 */
-	memblock_enforce_memory_limit(mem_start - PAGE_OFFSET);
+	memblock_enforce_memory_limit(-PAGE_OFFSET);
 
 	/* Reserve from the start of the kernel to the end of the kernel */
 	memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);