jack: Avoid call to snd_pcm_avail_update() from JACK thread

jack: Avoid call to snd_pcm_avail_update() from JACK thread

snd_pcm_avail_update() can call snd_pcm_jack_stop() but
snd_pcm_jack_stop() should not be called by the JACK thread.
It should only be called by the thread how has called
snd_pcm_jack_start().

In addition the execution of snd_pcm_avail_update() can take a while.
Therefore it should not be called by the JACK thread to not block this
thread.

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>