f2fs-tools: create sparse file first before stat

We must create a sparse file first before calling stat().

Fixes: eb9d8037 ("f2fs-tools: avoid mounting f2fs if tools already open the device")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>