f2fs: fix the wrong stat count of calling gc

With a partition which was formated as multi segments in one section,
we stated incorrectly for count of gc operation.

e.g., for a partition with segs_per_sec = 4

cat /sys/kernel/debug/f2fs/status

GC calls: 208 (BG: 7)
  - data segments : 104 (52)
  - node segments : 104 (24)

GC called count should be (104 (data segs) + 104 (node segs)) / 4 = 52,
rather than 208. Fix it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>