pseries/papr-hvpipe: Prevent kernel stack memory leak to userspace

In the Linux kernel, the following vulnerability has been resolved: pseries/papr-hvpipe: Prevent kernel stack memory leak to userspace The hdr variable is allocated on the stack and only hdr.version and hdr.flags are initialized explicitly. Because the struct papr_hvpipe_hdr contains reserved padding bytes (reserved[3] and reserved2[40]), these could leak the uninitialized bytes to userspace after copy_to_user(). This patch fixes that by initializing the whole struct to 0.