f2fs: introduce get_checkpoint_version for cleanup

There exists almost same codes when get the value of pre_version
and cur_version in function validate_checkpoint, this patch adds
get_checkpoint_version to clean up redundant codes.

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>