disk: inherit partition size when converting / to btrfs

Prior this commit, if a blueprint mountpoint grew / and the / partition
was converted to btrfs later, we didn't carry the enlarged size over.
This commit carries the former partition size to the created btrfs
subvolume.

A test will be added in the following commit.

pkg/disk/partition_table.go

@@ -781,6 +781,7 @@ func (pt *PartitionTable) ensureBtrfs() error {
 					Mountpoint: "/",
 					Compress:   DefaultBtrfsCompression,
 					ReadOnly:   opts.ReadOnly(),
+					Size:       part.Size,
 				},
 			},
 		}