f2fs: reset default idle interval value

The default value of idle interval is 2 mins, but for most time when
screen shutdown, there are still operations during the 2 mins interval,
and gc's sleep time is about 30 secs to 60 secs, so there is almost no
chance for GC thread to do garbage collecting.

Set default value of idle interval value from 2 mins to 5 secs for
fixing.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>