erofs-utils: fix `--blobdev=X`

Create one if the file doesn't exist.

Fixes: b6b741d8daaf ("erofs-utils: lib: get rid of tmpfile()")
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20250427021555.99648-1-hsiangkao@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>