ALSA: bcd2000: Fix race between rawmidi and disconnect

In the Linux kernel, the following vulnerability has been resolved: ALSA: bcd2000: Fix race between rawmidi and disconnect Although we tried to fix the potential UAF issues at USB disconnect on bcd2000 driver, there is still an overlooked case -- namely, when a rawmidi trigger callback has been already running at USB disconnect handling, the in-flight function (e.g. bcd2000_midi_send()) could still access the URB, because the previous URB NULL-check & clearance was considered only for the URB complete callbacks, but not about the parallel rawmidi operations. For addressing the race, this patch introduced a new spinlock that covers each rawmidi operation as well as the rawmidi handling in the complete callback. The URB is cleared with the lock, so it guarantees that the pending rawmidi task already finished or a NULL check is effective.