msec and grep option issue when I run AWS EFS as PVC

[sysnet4admin]

Issue title:

Mostly okay but when I run AWS EFA as PVC, write Latency cannot show up properly.

1. msec
When msec have occur, Dbench summary could not show up Average Latency (usec) like below.

==================
= Dbench Summary =
==================
Random Read/Write IOPS: 25.9k/7371. BW: 306MiB/s / 100MiB/s
Average Latency (usec) Read/Write: 2651.74 / 
Sequential Read/Write: 313MiB/s / 102MiB/s
Mixed Random Read/Write IOPS: 7408 / 2457

grep cannot figure it out.

Thus I change the code like this.

 READ_LATENCY_VAL=$(echo "$READ_LATENCY"|grep ' lat.*avg'|grep -Eoi 'avg=[\b 0-9.]+'|cut -d'=' -f2)

2. grep -P option for unit
usec is not dynamic variable. so changed like below:

 READ_LATENCY_UNIT=$(echo "$READ_LATENCY"|grep ' lat.*avg'|grep -oP '(?<=lat )[^ ]*'|grep -Eoi '[a-z]+')

current grep doesn't have grep -P(perl) type.

Run status group 0 (all jobs):
   READ: bw=6502KiB/s (6658kB/s), 6502KiB/s-6502KiB/s (6658kB/s-6658kB/s), io=95.3MiB (99.9MB), run=15003-15003msec
grep: unrecognized option: P
BusyBox v1.25.1 (2016-10-26 16:15:20 GMT) multi-call binary.

Thus I rebuild it from alpine.

Current output (run AWS EFS)

==================
= Dbench Summary =
==================
Random Read/Write IOPS: 24.5k/7514. BW: 298MiB/s / 100MiB/s
Average Latency Read/Write: 2449.78(usec) / 8069.02(usec)
Sequential Read/Write: 302MiB/s / 102MiB/s
Mixed Random Read/Write IOPS: 7414 / 2493

Here is the source which is modified above.
https://github.com/sysnet4admin/dbench

I would like to PR but...it is not working in here....