ALSA: line6: Fix racy access to midibuf

There can be concurrent accesses to line6 midibuf from both the URB
completion callback and the rawmidi API access.  This could be a cause
of KMSAN warning triggered by syzkaller below (so put as reported-by
here).

This patch protects the midibuf call of the former code path with a
spinlock for avoiding the possible races.

Reported-by:  <syzbot+78eccfb8b3c9a85fc6c5@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/00000000000000949c061df288c5@google.com
Cc: <stable@vger.kernel.org>
Link: https://patch.msgid.link/20240805130129.10872-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>