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.