snapshot: add Usage method to Snapshotter

To allow the querying of usage for snapshots, we define a new method on
the snapshotter to query the resources in use by a single snapshot.
Conversely, it can be said that if the snapshot was deleted, the
reported amount of usage would be recovered.

There are few problems with this model in the implementation of btrfs
that need to be worked out. In btrfs, it is hard to resolve the amount
of data usage with the use of quotas but these may report valuables that
are incompatible with the model.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

fs/du_unix.go

@@ -0,0 +1,42 @@
+// +build !windows
+
+package fs
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+func diskUsage(roots ...string) (Usage, error) {
+	type inode struct {
+		// TODO(stevvooe): Can probably reduce memory usage by not tracking
+		// device, but we can leave this right for now.
+		dev, ino uint64
+	}
+
+	var (
+		size   int64
+		inodes = map[inode]struct{}{} // expensive!
+	)
+
+	for _, root := range roots {
+		if err := filepath.Walk(root, func(path string, fi os.FileInfo, err error) error {
+			if err != nil {
+				return err
+			}
+
+			stat := fi.Sys().(*syscall.Stat_t)
+			inodes[inode{dev: stat.Dev, ino: stat.Ino}] = struct{}{}
+			size += fi.Size()
+			return nil
+		}); err != nil {
+			return Usage{}, err
+		}
+	}
+
+	return Usage{
+		Inodes: int64(len(inodes)),
+		Size:   size,
+	}, nil
+}