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

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

cfq and bfq have 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 28f4197e5d47 ("block: disable preemption before
using sched_clock()").

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry-picked from 84c7afcebed913c93d50f116b046b7f0d8ec0cdc)
[nc: only backport cfq changes]
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Harsh Shandilya <msfjarvis@gmail.com>