powerpc/nvram: Ensure that the partition header/block size is right

Use BUILD_BUG_ON to ensure the structure representing a partition
header have the right size.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/nvram_64.c

@@ -565,6 +565,8 @@ static int __init nvram_init(void)
 	int error;
 	int rc;
 	
+	BUILD_BUG_ON(NVRAM_BLOCK_LEN != 16);
+
 	if (ppc_md.nvram_size == NULL || ppc_md.nvram_size() <= 0)
 		return  -ENODEV;