mmc: card: change long_sequential_test time measurements to ktime

Change time measurements in long_sequential_test from jiffies to ktime,
and make the relevant change in test-iosched infrastructure.

In long_sequential_test we measure throughput, and the jiffies resolution
is not sensitive enough for this calculation.

Change-Id: If7c9a03c687f61996609c014e056bcd7132b9012
Signed-off-by: Lee Susman <lsusman@codeaurora.org>