ALSA: seq: oss: Fix data-race for max_midi_devs access

ALSA OSS sequencer refers to a global variable max_midi_devs at
creating a new port, storing it to its own field.  Meanwhile this
variable may be changed by other sequencer events at
snd_seq_oss_midi_check_exit_port() in parallel, which may cause a data
race.

OTOH, this data race itself is almost harmless, as the access to the
MIDI device is done via get_mdev() and it's protected with a refcount,
hence its presence is guaranteed.

Though, it's sill better to address the data-race from the code sanity
POV, and this patch adds the proper spinlock for the protection.
Reported-by: Abhishek Shah <abhishek.shah@columbia.edu>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CAEHB2493pZRXs863w58QWnUTtv3HHfg85aYhLn5HJHCwxqtHQg@mail.gmail.com
Link: https://lore.kernel.org/r/20220823072717.1706-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/oss/seq_oss_midi.c

@@ -270,7 +270,9 @@ snd_seq_oss_midi_clear_all(void)
 void
 snd_seq_oss_midi_setup(struct seq_oss_devinfo *dp)
 {
+	spin_lock_irq(&register_lock);
 	dp->max_mididev = max_midi_devs;
+	spin_unlock_irq(&register_lock);
 }
 
 /*