ALSA: line6: Zero-initialize message buffers

For shutting up spurious KMSAN uninit-value warnings, just replace
kmalloc() calls with kzalloc() for the buffers used for
communications.  There should be no real issue with the original code,
but it's still better to cover.

Reported-by:  <syzbot+7fb05ccf7b3d2f9617b3@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/r/00000000000084b18706150bcca5@google.com
Message-ID: <20240402063628.26609-1-tiwai@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>