Werk #5708: df: Fix incorrect values being reported on devices with btrfs

Component	Checks & agents
Title	df: Fix incorrect values being reported on devices with btrfs
Date	Jan 28, 2018
Level	Prominent Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.5.0i3 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

This fixes a bug introduced with Werk #4806.

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