f2fs: maintain six open zones for zoned devices

To keep six open zone constraints, make them not to be open over six
open zones.

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