net-timestamp: fix allocation error in test

A buffer is incorrectly zeroed to the length of the pointer. If
cfg_payload_len < sizeof(void *) this can overwrites unrelated memory.
The buffer contents are never read, so no need to zero.

Fixes: 8fe2f761

 ("net-timestamp: expand documentation")

Reported-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>