f2fs: support checkpoint error injection

This patch adds to support checkpoint error injection in f2fs for testing
fatal error tolerance, it will be useful that it can simulate abnormal
power off by f2fs itself instead of calling godown ioctl by running apps.

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