Handle older versions of the Linux kernel in a safer way

The Linux kernel does argument splitting after parameter expansion,
leading to an attacker on old kernels being able control the owner
variable, leading to the creation of bogus root-owned directories.

Fixes: https://bugs.debian.org/924398


Reported-by: Jakub Wilk <jwilk@jwilk.net>
Reported-in: <20190312145043.jxjoj66kqssptolr@jwilk.net>