cfq-iosched: Convert slice_resid from u64 to s64

cfq-iosched: Convert slice_resid from u64 to s64

slice_resid can be both positive and negative. Commit 9a7f38c42c2b
(cfq-iosched: Convert from jiffies to nanoseconds) converted it from
long to u64. Although this did not introduce any functional regression
(the operations just overflow and the result was fine), it is certainly
wrong and could cause issues in future. So convert the type to more
appropriate s64.

Fixes: 9a7f38c42c2b92391d9dabaf9f51df7cfe5608e4
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>