ALSA: rawmidi: Access runtime->avail always in spinlock

ALSA: rawmidi: Access runtime->avail always in spinlock

The runtime->avail field may be accessed concurrently while some
places refer to it without taking the runtime->lock spinlock, as
detected by KCSAN.  Usually this isn't a big problem, but for
consistency and safety, we should take the spinlock at each place
referencing this field.

Reported-by: syzbot+a23a6f1215c84756577c@syzkaller.appspotmail.com
Reported-by: syzbot+3d367d1df1d2b67f5c19@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/20201206083527.21163-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>