Have <Plugin df> ignore squashfs by default

[Marius Gedminas]

I've noticed today that collectd tracks disk usage of every version of every snap, since those are squashfs mounts. There are rather a lot of those:

$ ls -ld /var/lib/collectd/rrd/$(hostname -f)/df-snap*|wc -l
262

I suggest adding

    FSType squashfs

to the <Plugin df> block in the default /etc/collectd/collectd.conf.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: collectd 5.9.2.g-1ubuntu5
ProcVersionSignature: Ubuntu 5.4.0-67.75-generic 5.4.94
Uname: Linux 5.4.0-67-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: skip
Date: Tue Mar 16 17:37:45 2021
InstallationDate: Installed on 2016-09-10 (1648 days ago)
InstallationMedia: Ubuntu-GNOME 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160720)
SourcePackage: collectd
UpgradeStatus: Upgraded to focal on 2020-04-26 (324 days ago)