engines/null: fill issue_time during commit

engines/null: fill issue_time during commit

For iodepth > 1, the null ioengine is an asynchronous ioengine, yet it
does not report submission latency. This patch adds code for setting
issue_time in order to calculate submission latency.

The extra work does reduce performance for iodepth > 1 but this
reduction can be mitigated with options like gtod_reduce.

- there are no differeces at qd 1 (5837K vs 5864K)
- default options at qd 256 shows an 8% IOPS reduction (3583K vs 3303K).
- gtod_reduce=1 at qd 256 shows a 3% IOPS reduction (5119K vs 4966K).

$ ../fio-canonical/fio --name=test --ioengine=null --size=1T --norandommap --ramp_time=5s --runtime=15s --cpus_allowed=1
test: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=1
fio-3.30-73-gd622
<snip>
  read: IOPS=5837k, BW=22.3GiB/s (23.9GB/s)(334GiB/15001msec)
<snip>

$ ./fio --name=test --ioengine=null --size=1T --norandommap --ramp_time=5s --runtime=15s --cpus_allowed=1
test: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=1
fio-3.30-74-gcfbc2
<snip>
  read: IOPS=5864k, BW=22.4GiB/s (24.0GB/s)(336GiB/15001msec)
<snip>

$ ../fio-canonical/fio --name=test --ioengine=null --size=1T --norandommap --iodepth=256 --ramp_time=5s --runtime=15s --cpus_allowed=1
test: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=256
fio-3.30-73-gd622
<snip>
  read: IOPS=3583k, BW=13.7GiB/s (14.7GB/s)(205GiB/15001msec)
<snip>

$ ./fio --name=test --ioengine=null --size=1T --norandommap --iodepth=256 --ramp_time=5s --runtime=15s --cpus_allowed=1
test: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=256
fio-3.30-74-gcfbc2
<snip>
  read: IOPS=3303k, BW=12.6GiB/s (13.5GB/s)(189GiB/15001msec)
<snip>

$ ../fio-canonical/fio --name=test --ioengine=null --size=1T --norandommap --iodepth=256 --ramp_time=5s --runtime=15s --cpus_allowed=1 --gtod_reduce=1
test: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=256
fio-3.30-73-gd622
<snip>
  read: IOPS=5119k, BW=19.5GiB/s (21.0GB/s)(293GiB/15001msec)
<snip>

$ ./fio --name=test --ioengine=null --size=1T --norandommap --iodepth=256 --ramp_time=5s --runtime=15s --cpus_allowed=1 --gtod_reduce=1
test: (g=0): rw=read, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=null, iodepth=256
fio-3.30-74-gcfbc2
<snip>
  read: IOPS=4966k, BW=18.9GiB/s (20.3GB/s)(284GiB/15001msec)
<snip>

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>