um: register power-off handler

Otherwise we always get

 reboot: Power off not available: System halted instead

which is really quite pointless.

Link: https://patch.msgid.link/20240703173839.fcbb538c6686.I3d333f4773cff93c4337c4d128ee0b1b501b3dfa@changeidAcked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/kernel/reboot.c

@@ -59,3 +59,18 @@ void machine_halt(void)
 {
 	machine_power_off();
 }
+
+static int sys_power_off_handler(struct sys_off_data *data)
+{
+	machine_power_off();
+	return 0;
+}
+
+static int register_power_off(void)
+{
+	register_sys_off_handler(SYS_OFF_MODE_POWER_OFF,
+				 SYS_OFF_PRIO_DEFAULT,
+				 sys_power_off_handler, NULL);
+	return 0;
+}
+__initcall(register_power_off);