Add loopback_darwin.

fuse/pathfs/loopback_darwin.go

+package fuse
+
+import (
+	"syscall"
+)
+
+func (fs *LoopbackFileSystem) StatFs(name string) *StatfsOut {
+	s := syscall.Statfs_t{}
+	err := syscall.Statfs(fs.GetPath(name), &s)
+	if err == nil {
+		return &StatfsOut{
+			Blocks: s.Blocks,
+			Bsize:  uint32(s.Bsize),
+			Bfree:  s.Bfree,
+			Bavail: s.Bavail,
+			Files:  s.Files,
+			Ffree:  s.Ffree,
+		}
+	}
+	return nil
+}