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

sound/core/memalloc.c

@@ -135,16 +135,17 @@ int snd_dma_alloc_pages(int type, struct device *device, size_t size,
 	dmab->dev.type = type;
 	dmab->dev.dev = device;
 	dmab->bytes = 0;
+	dmab->area = NULL;
+	dmab->addr = 0;
+	dmab->private_data = NULL;
 	switch (type) {
 	case SNDRV_DMA_TYPE_CONTINUOUS:
 		gfp = snd_mem_get_gfp_flags(device, GFP_KERNEL);
 		dmab->area = alloc_pages_exact(size, gfp);
-		dmab->addr = 0;
 		break;
 	case SNDRV_DMA_TYPE_VMALLOC:
 		gfp = snd_mem_get_gfp_flags(device, GFP_KERNEL | __GFP_HIGHMEM);
 		dmab->area = __vmalloc(size, gfp);
-		dmab->addr = 0;
 		break;
 #ifdef CONFIG_HAS_DMA
 #ifdef CONFIG_GENERIC_ALLOCATOR
@@ -171,8 +172,6 @@ int snd_dma_alloc_pages(int type, struct device *device, size_t size,
 #endif
 	default:
 		pr_err("snd-malloc: invalid device type %d\n", type);
-		dmab->area = NULL;
-		dmab->addr = 0;
 		return -ENXIO;
 	}
 	if (! dmab->area)