ALSA: pcm: Fix memory leak at closing a stream without hw_free

ALSA PCM core recently introduced a new managed PCM buffer allocation
mode that does allocate / free automatically at hw_params and
hw_free.  However, it overlooked the code path directly calling
hw_free PCM ops at releasing the PCM substream, and it may result in a
memory leak as spotted by syzkaller when no buffer preallocation is
used (e.g. vmalloc buffer).

This patch papers over it with a slight refactoring.  The hw_free ops
call and relevant tasks are unified in a new helper function, and call
it from both places.

Fixes: 0dba808e ("ALSA: pcm: Introduce managed buffer allocation mode")
Reported-by:  <syzbot+30edd0f34bfcdc548ac4@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200129195907.12197-1-tiwai@suse.de


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