struct nvme_hmb_desc contains a pad field which was not getting
initialized before being synced. This doesn't have much consequence but
triggers a report from KMSAN, which verifies that host-filled DMA memory
is initialized before it is made visible to the device. So, let's just
initialize it properly.
Reported by: KMSAN
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.