rbd: use kstrndup() in rbd_header_from_disk()

	/* Allocate this now to avoid having to handle failure below */

	if (first_time) {

		size_t len;

		len = strnlen(ondisk->object_prefix,

				sizeof (ondisk->object_prefix));

		object_prefix = kmalloc(len + 1, GFP_KERNEL);

		object_prefix = kstrndup(ondisk->object_prefix,

					 sizeof(ondisk->object_prefix),

					 GFP_KERNEL);

		if (!object_prefix)

			return -ENOMEM;

		memcpy(object_prefix, ondisk->object_prefix, len);

		object_prefix[len] = '\0';

	}

	/* Allocate the snapshot context and fill it in */