f2fs-tools: avoid mounting f2fs if tools already open the device

If the block device is opened by tools, F2FS should not be mounted.
Especially when fsck is running, errors unexpected may happen. So if
tools open a block device, we give it the O_EXCL flag to make sure
the block device is opened exclusivly.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>