f2fs: Fix over-estimating free section during FG GC

There was a bug that finishing FG GC unconditionally because free sections
are over-estimated after checkpoint in FG GC.
This patch initializes sec_freed by every checkpoint in FG GC.

Signed-off-by: Yonggil Song <yonggil.song@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>