ALSA: memalloc: Initialize all fields of snd_dma_buffer properly

Some fields in snd_dma_buffer aren't touched in snd_dma_alloc_pages()
and might be left uninitialized.  Let's clear all fields properly, so
that we can use a NULL check (e.g. dmab->private_data) as conditional
in a later patch.

Link: https://lore.kernel.org/r/20200615160045.2703-3-tiwai@suse.de


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