Minor formatting error on Fedora 27

[JamesReynolds]

+----------------------------+    +----------------------------+
| SOURCE HOST                |    | DESTINATION HOST           |
| IP:   xxxxxxxxxxxxx        |    | IP:  xxxxxxxxxx            |
| ARP:  xxxxxxxxxxxxxxxxx    || ARP:  xxxxxxxxxxxxxxxxx    |    |                            |
+----------------------------+    +----------------------------+
                  |                             |
                  |               +----------------------------+
                  |               | ROUTER TOWARDS DESTINATION |
                  |               | IP:   xxxxxxxxxxxxx        |
                  |               | ARP:  xxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxx|
                  |               +----------------------------+
                  |                             |               
            --+---+-----------------------------+---
                  |
...

Looks like src_gateway_arp is coming out as:

"xxxxxxxxxxxxxxxxx\n
xxxxxxxxxxxxxxxxx"

so changing to:
printf '| ARP: %-18s |' "${src_gateway_arp:0:17}"
works.