ALSA: rawmidi: Change resized buffers atomically

commit 39675f7a7c7e7702f7d5341f1e0d01db746543a0 upstream.

The SNDRV_RAWMIDI_IOCTL_PARAMS ioctl may resize the buffers and the
current code is racy.  For example, the sequencer client may write to
buffer while it being resized.

As a simple workaround, let's switch to the resized buffer inside the
stream runtime lock.

Reported-by:  <syzbot+52f83f0ea8df16932f7f@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>