ALSA: info: Use kvzalloc() for a temporary write buffer

We used to use kmalloc (more exactly, krealloc()) for creating and
growing the temporary buffer for text proc write.  It can grow up to
16kB, and it's already a bit doubtful whether it's always safe to use
kmalloc().  With the recent addition of kvmalloc(), we can have a
better chance for succeed of memory allocation, so let's switch to
that new API.

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