shutdown: guard against read-only /run

remount the switch rooted /run writeable again.

modules.d/99shutdown/module-setup.sh

@@ -14,7 +14,7 @@ depends() {
 # called by dracut
 install() {
     local _d
-    inst_multiple umount poweroff reboot halt losetup
+    inst_multiple umount poweroff reboot halt losetup stat
     inst_multiple -o kexec
     inst "$moddir/shutdown.sh" "$prefix/shutdown"
     [ -e "${initdir}/lib" ] || mkdir -m 0755 -p ${initdir}/lib

modules.d/99shutdown/shutdown.sh

@@ -12,6 +12,10 @@ export TERM=linux
 export PATH=/usr/sbin:/usr/bin:/sbin:/bin
 . /lib/dracut-lib.sh
 
+if [ "$(stat -c '%T' -f /)" = "tmpfs" ]; then
+    mount -o remount,rw /
+fi
+
 mkdir /oldsys
 for i in sys proc run dev; do
     mkdir /oldsys/$i