Werk #7465: diskstats: Add support for kernel 4.18+

Component	Checks & agents
Title	diskstats: Add support for kernel 4.18+
Date	May 15, 2019
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.6.0b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME) 1.5.0p17 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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