sysutils/node_exporter: fix meminfo collector

meminfo collector failed with:

node_exporter[29302]: ts=2023-02-04T09:06:27.328Z caller=collector.go:169
level=error msg="collector failed" name=meminfo duration_seconds=0.000127617
err="couldn't get memory: cannot allocate memory"

This patch contains a fix for the meminfo collector, it has also been submitted
upstream.

PR: 269154
Reported by: David O'Rourke <dor.bsd@xm0.uk>