f2fs: write checkpoint during FG_GC

If there's not enough free sections each of which consistis of large segments,
we can hit no free section for upcoming section allocation. Let's reclaim some
prefree segments by writing checkpoints.

Signed-off-by: Byungki Lee <dominicus79@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>