usx2y drivers may expose the allocated pages via mmap, but it performs
zero-clear only for the struct size, not aligned with the page size.
This leaves out some uninitialized trailing bytes.

This patch fixes the clearance to cover all memory that are exposed to
user-space.

Link: https://lore.kernel.org/r/20210517131545.27252-5-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>