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 aa795a966fc0
(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: aa795a966fc0e97073dc23c0ef171a487c11a618
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>