fsck.f2fs: allow to fsck readonly image w/ -f option

To keep line with e2fsprogs, let's allow to fsck mounted image as
readonly w/ -f option.

Reported-by: Perfect Gentleman <perfect007gentleman@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
3 files changed