write out the backup header too

Now that we actually have the real backup header, we can safely write it
out without having the wrong LBA pointers and other such issues that
would come from writing the primary header to the backup header (ask me
how I know)

gpt-utils.cpp

@@ -774,6 +774,21 @@ int gpt_disk_commit(struct gpt_disk *disk)
 			__func__);
 		goto error;
 	}
+
+	// Write the backup header
+	if (gpt_set_header(disk->hdr_bak, fd, SECONDARY_GPT) != 0) {
+		fprintf(stderr, "%s: Failed to update backup GPT header\n",
+			__func__);
+		goto error;
+	}
+	LOGD("%s: Writing back backup partition array\n", __func__);
+	// Write back the backup partition array
+	if (gpt_set_pentry_arr(disk->hdr_bak, fd, disk->pentry_arr_bak)) {
+		fprintf(stderr,
+			"%s: Failed to write backup GPT partition arr\n",
+			__func__);
+		goto error;
+	}
 	fsync(fd);
 	close(fd);
 	return 0;