Fix systemd graphs using wrong rrd filename variable

[sarabveer]

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This pull request has been mentioned on LibreNMS Community. There might be relevant details there:

https://community.librenms.org/t/issue-with-systemd-graph-showing-no-data-file/24156/2

[sarabveer]

Broken with this commit: 0998592#diff-8b768feed8fa315efa39e9aa7f7d26064b7ab31a397e30e736dc592ce0a5b8f8

Variable name was changed, but the if statement not updated.

[bnerickson]

lgtm