rpmsg: virtio: Replace deprecated strncpy with strscpy/_pad

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

This patch replaces 3 callsites of strncpy().

The first two populate the destination buffer `nsm.name` -- which we
expect to be NUL-terminated based on their use with format strings.

Firstly, as I understand it, virtio_rpmsg_announce_create() creates an
rpmsg_ns_msg and sends via:

virtio_rpmsg_bus.c:
336: err = rpmsg_sendto(rpdev->ept, &nsm, sizeof(nsm), RPMSG_NS_ADDR);

... which uses:
virtio_rpmsg_sendto() -> rpmsg_send_offchannel_raw()

... which copies its data into an rpmsg_hdr `msg` in virtio_rpmsg_bus.c
618: memcpy(msg->data, data, len);

This callback is invoked when a message is received from the remote
processor:

rpmsg_ns.c:
30: /* invoked when a name service announcement arrives */
31: static int rpmsg_ns_cb(struct rpmsg_device *rpdev, void *data, int len,
32: 		       void *priv,...