BACKPORT: block: use ktime_get_ns() instead of sched_clock() for cfq

cfq has some internal fields that use sched_clock() which can
trivially use ktime_get_ns() instead. Their timestamp fields in struct
request can also use ktime_get_ns(), which resolves the 8 year old
comment added by commit 28f4197e ("block: disable preemption before
using sched_clock()").

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Yaroslav Furman <yaro330@gmail.com>
Signed-off-by: Adam W. Willis <return.of.octobot@gmail.com>
Signed-off-by: Yaroslav Furman <yaro330@gmail.com>