f2fs: fix to clear CP_TRIMMED_FLAG

Once CP_TRIMMED_FLAG is set, after a reboot, we will never issue discard
before LBA becomes invalid again, fix it by clearing the flag in
checkpoint without CP_TRIMMED reason.

Fixes: 1f43e2ad7bff ("f2fs: introduce CP_TRIMMED_FLAG to avoid unneeded discard")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>