Default age threshold value is missed to set, fix it.

Fixes: 093749e2 ("f2fs: support age threshold based garbage collection")
Reported-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>