f2fs: new helper cur_cp_crc() getting crc in f2fs_checkpoint

There are four places that getting the crc value in f2fs_checkpoint,
just add a new helper cur_cp_crc for them.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>