gpt: point alternate PartitionEntryLBA to alternate GPT table

Currently both tables point to the primary GPT partition entry
array which doesn't make a lot of sense. Pass "primary" to
t.partitionArraySector() to get the right address for each of
the tables instead.

partition/gpt/table.go

@@ -295,7 +295,7 @@ func (t *Table) toGPTBytes(primary bool) ([]byte, error) {
 	copy(b[56:72], bytesToUUIDBytes(guid[0:16]))
 
 	// starting LBA of array of partition entries
-	binary.LittleEndian.PutUint64(b[72:80], t.partitionArraySector(true))
+	binary.LittleEndian.PutUint64(b[72:80], t.partitionArraySector(primary))
 
 	// how many entries?
 	binary.LittleEndian.PutUint32(b[80:84], uint32(t.partitionArraySize))