f2fs: simplify do_checkpoint

For each loop add a local curseg_info pointer insted of looking it up
for each of the three fields.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>