cfq-iosched: Charge at least 1 jiffie instead of 1 ns

Commit 9a7f38c4 (cfq-iosched: Convert from jiffies to nanoseconds)
could result in charging just 1 ns to a cgroup submitting IO instead of 1
jiffie we always charged before. It is arguable what is the right amount
to change but for now lets retain the old behavior of always charging at
least one jiffie.

Fixes: 9a7f38c4
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>