This patch adds the vmalloc buffer support to ALSA memalloc core.  A
new type, SNDRV_DMA_TYPE_VMALLOC was added.

The vmalloc buffer has been already supported in the PCM via a few own
helper functions, but the user sometimes get confused and misuse
them.  With this patch, the whole buffer management is integrated into
the memalloc core, so they can be used in a sole common way.

Link: https://lore.kernel.org/r/20191105080138.1260-3-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>