f2fs: skip GC if possible when checkpoint disabling

If the number of unusable blocks is not larger than
unusable capacity, we can skip GC when checkpoint
disabling.

Signed-off-by: Weichao Guo <guoweichao@oppo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
[Jaegeuk Kim: Fix missing gc_mode assignment]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>