f2fs: prevent an f2fs_gc loop during disable_checkpoint

Don't get stuck in the f2fs_gc loop while disabling checkpoint. Instead, we have
a time-based management.

Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>