ALSA: seq: Fix KCSAN data-race warning at snd_seq_fifo_poll_wait()

snd_seq_fifo_poll_wait() evaluates f->cells without locking after
poll_wait(), and KCSAN doesn't like it as it appears to be a
data-race. Although this doesn't matter much in practice as the value
is volatile, it's still better to address it for the mind piece.

Wrap it with f->lock spinlock for avoiding the potential data race.

Reported-by: [e-mail]
Link: https://syzkaller.appspot.com/bug?extid=c3dbc239259940ededba
Signed-off-by: Takashi Iwai <[e-mail]>

sound/core/seq/seq_fifo.c

···
 			   poll_table *wait)
 {
 	poll_wait(file, &f->input_sleep, wait);
+	guard(spinlock_irq)(&f->lock);
 	return (f->cells > 0);
 }