f2fs: set dirty state for filesystem only when updating meta data

We don't guarantee integrity of user data after checkpoint, since we only
guarantee meta data integrity for data consistency of filesystem.

Due to above reason, we only need to set fs as dirty when meta data is
updated, so that we can skip writing checkpoint in some case of non-meta
data is updated.

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