ALSA: rawmidi: Initialize allocated buffers

syzbot reported the uninitialized value exposure in certain situations
using virmidi loop.  It's likely a very small race at writing and
reading, and the influence is almost negligible.  But it's safer to
paper over this just by replacing the existing kvmalloc() with
kvzalloc().

Reported-by:  <syzbot+194dffdb8b22fc5d207a@syzkaller.appspotmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>