USB: Proper handling of Race Condition when two USB class drivers try to call init_usb_class simultaneously
There is race condition when two USB class drivers try to call
init_usb_class at the same time and leads to crash.
code path: probe->usb_register_dev->init_usb_class
To solve this, mutex locking has been added in init_usb_class() and
destroy_usb_class().
As pointed by Alan, removed "if (usb_class)" test from destroy_usb_class()
because usb_class can never be NULL there.