thunderbolt: Fix use-after-free in remove_unplugged_switch()

Paulian reported a crash that happens when a dock is unplugged during
hibernation:

[78436.228217] thunderbolt 0-1: device disconnected
[78436.228365] BUG: kernel NULL pointer dereference, address: 00000000000001e0
...
[78436.228397] RIP: 0010:icm_free_unplugged_children+0x109/0x1a0
...
[78436.228432] Call Trace:
[78436.228439]  icm_rescan_work+0x24/0x30
[78436.228444]  process_one_work+0x1a3/0x3a0
[78436.228449]  worker_thread+0x30/0x370
[78436.228454]  ? process_one_work+0x3a0/0x3a0
[78436.228457]  kthread+0x13d/0x160
[78436.228461]  ? kthread_park+0x90/0x90
[78436.228465]  ret_from_fork+0x1f/0x30

This happens because remove_unplugged_switch() calls tb_switch_remove()
that releases the memory pointed by sw so the following lines reference
to a memory that might be released already.

Fix this by saving pointer to the parent device before calling
tb_switch_remove().
Reported-by: Paulian Bogdan Marinca <paulian@marinca.net>
Fixes: 4f7c2e0d ("thunderbolt: Make sure device runtime resume completes before taking domain lock")
Cc: stable@vger.kernel.org
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/thunderbolt/icm.c

@@ -1976,7 +1976,9 @@ static int complete_rpm(struct device *dev, void *data)
 
 static void remove_unplugged_switch(struct tb_switch *sw)
 {
-	pm_runtime_get_sync(sw->dev.parent);
+	struct device *parent = get_device(sw->dev.parent);
+
+	pm_runtime_get_sync(parent);
 
 	/*
 	 * Signal this and switches below for rpm_complete because
@@ -1987,8 +1989,10 @@ static void remove_unplugged_switch(struct tb_switch *sw)
 	bus_for_each_dev(&tb_bus_type, &sw->dev, NULL, complete_rpm);
 	tb_switch_remove(sw);
 
-	pm_runtime_mark_last_busy(sw->dev.parent);
-	pm_runtime_put_autosuspend(sw->dev.parent);
+	pm_runtime_mark_last_busy(parent);
+	pm_runtime_put_autosuspend(parent);
+
+	put_device(parent);
 }
 
 static void icm_free_unplugged_children(struct tb_switch *sw)