pcm: direct: fix race on clearing timer events

pcm: direct: fix race on clearing timer events

snd_timer handling is racy: plugins clear timer queue if avail_min
is not reached to force a sleep on timer. The race can happen if
the expected event arrives in between the avail check and the
clearing of pending events. If this race happens, the user will
unnecessarily wait for one more timer event. On low latency/realtime
streams this can lead to xruns and must be avoided.

As a fix we recheck avail after having cleared poll events.

Signed-off-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Mounesh Sutar <sutar.mounesh@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>