f2fs: write checkpoint during FG_GC

[ Upstream commit a9163b94 ]

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>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>