systemd/dracut-cmdline.sh: unset "UNSET" $root

https://bugzilla.redhat.com/show_bug.cgi?id=1365667
caused by commit 9aa224cc

modules.d/98systemd/dracut-cmdline.sh

@@ -26,7 +26,7 @@ source_conf /etc/conf.d
 # between the case where it was set to the empty string and the case where it
 # wasn't specified at all.
 if ! root="$(getarg root=)"; then
-    root='UNSET'
+    root_unset='UNSET'
 fi
 
 rflags="$(getarg rootflags=)"
@@ -50,7 +50,7 @@ source_hook cmdline
 
 [ -f /lib/dracut/parse-resume.sh ] && . /lib/dracut/parse-resume.sh
 
-case "$root" in
+case "${root}${root_unset}" in
     block:LABEL=*|LABEL=*)
         root="${root#block:}"
         root="$(echo $root | sed 's,/,\\x2f,g')"
@@ -76,7 +76,7 @@ case "$root" in
         rootok=1 ;;
 esac
 
-[ -z "$root" ] && die "Empty root= argument"
+[ -z "${root}${root_unset}" ] && die "Empty root= argument"
 [ -z "$rootok" ] && die "Don't know how to handle 'root=$root'"
 
 export root rflags fstype netroot NEWROOT