Fix header_interval minimum value

It doesn't make sense to have a value of header size or less, since
then there's no room for actual data. Fio even crashes with a value
less than the header size.

Lets just make the minimum twice the header size, seems reasonable...

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/options.c b/options.c
index 130dc8c..01d2386 100644
--- a/options.c
+++ b/options.c
@@ -618,8 +618,8 @@
 		.name   = "header_interval",
 		.type   = FIO_OPT_STR_VAL_INT,
 		.off1   = td_var_offset(header_interval),
+		.minval	= 2 * sizeof(struct verify_header),
 		.help   = "Store buffer header every N bytes",
-		.def    = "0",
 	},
 	{
 		.name	= "header_offset",