fix(90multipath): create /etc/multipath only

The dropins path is `/etc/multipath/conf.d`, not
`/etc/multipath/multipath.conf.d`:

https://github.com/opensvc/multipath-tools/blob/686797b7d0da4fc24fdfcb0fc6364df78a0b232d/Makefile.inc#L100

But anyway, we don't need to create that directory. We only need to
create `/etc/multipath`.

While we're here, add a comment to explain this.

Author: jlebon

modules.d/90multipath/multipathd-configure.service

@@ -12,7 +12,8 @@ ConditionPathExists=!/etc/multipath.conf
 
 [Service]
 Type=oneshot
-ExecStartPre=-/usr/bin/mkdir -p /etc/multipath/multipath.conf.d
+# mpathconf requires /etc/multipath to already exist
+ExecStartPre=-/usr/bin/mkdir -p /etc/multipath
 ExecStart=/usr/sbin/mpathconf --enable
 
 [Install]

modules.d/90multipath/multipathd.sh

@@ -1,7 +1,8 @@
 #!/bin/sh
 
 if [ "$(getarg rd.multipath)" = "default" ] && [ ! -e /etc/multipath.conf ]; then
-    mkdir -p /etc/multipath/multipath.conf.d
+    # mpathconf requires /etc/multipath to already exist
+    mkdir -p /etc/multipath
     mpathconf --enable
 fi