usb: hub: take hub->hdev reference when processing from eventlist

During surprise device hotplug removal tests, it was observed that
hub_events may try to call usb_lock_device on a device that has already
been freed. Protect the usb_device by taking out a reference (under the
hub_event_lock) when hub_events pulls it off the list, returning the
reference after hub_events is finished using it.

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Suggested-by: David Bulkow <david.bulkow@stratus.com> for using kref
Suggested-by: Alan Stern <stern@rowland.harvard.edu> for placement
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>