kernel version: 4.1.12-1-default
OS: openSUSE Leap 42.1

Details:
  I try to reshape one raid1 to raid5 by using mdadm. And systemctl doesn't
escape the argument in Service file.

zlliu:~/mdadm # cat /usr/lib/systemd/system/mdadm-grow-continue@.service 
#  This file is part of mdadm.
#
#  mdadm is free software; you can redistribute it and/or modify it
#  under the terms of the GNU General Public License as published by
#  the Free Software Foundation; either version 2 of the License, or
#  (at your option) any later version.

[Unit]
Description=Manage MD Reshape on /dev/%I
DefaultDependencies=no

[Service]
ExecStart=/sbin/mdadm --grow --continue /dev/%I
StandardInput=null
StandardOutput=null
StandardError=null
KillMode=none

Test steps:
1. # dd if=/dev/zero of=test1 bs=1M count=100
   # dd if=/dev/zero of=test2 bs=1M count=100
   # dd if=/dev/zero of=test3 bs=1M count=100
   # losetup /dev/loop(0,1,2) test(1,2,3)
2. # mdadm -CR /dev/md1 -b internal -l1 -n2 /dev/loop[0-1]
   # mdadm --grow /dev/md1 -l5 -n3 -a /dev/loop2
You can check "systemctl status mdadm-grow-continue@md1.service" and
"journalctl -xn"

The same situation(include of mdadm source code and same testing steps) cannot
reproduce with SLES 12 SP2 and Tumbleweed.

Reproduce:  Always