ALSA: seq: oss: Fix racy open/close of MIDI devices

Although snd_seq_oss_midi_open() and snd_seq_oss_midi_close() can be
called concurrently from different code paths, we have no proper data
protection against races.  Introduce open_mutex to each seq_oss_midi
object for avoiding the races.

Reported-by: "Gong, Sishuai" <sishuai@purdue.edu>
Closes: https://lore.kernel.org/r/7DC9AF71-F481-4ABA-955F-76C535661E33@purdue.edu
Link: https://lore.kernel.org/r/20230612125533.27461-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>