Merge pull request #40652 from thaJeztah/19.03_backport_fix_backingfs

[19.03 backport] fix backingFs assignment
Upstream-commit: 6ed0f6ab78bf39f096bf2b3db8c498587b120d69
Component: engine

Author: AkihiroSuda

components/engine/daemon/graphdriver/overlay/overlay.go

@@ -136,6 +136,14 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, graphdriver.ErrNotSupported
 	}
 
+	fsMagic, err := graphdriver.GetFSMagic(testdir)
+	if err != nil {
+		return nil, err
+	}
+	if fsName, ok := graphdriver.FsNames[fsMagic]; ok {
+		backingFs = fsName
+	}
+
 	supportsDType, err := fsutils.SupportsDType(testdir)
 	if err != nil {
 		return nil, err

components/engine/daemon/graphdriver/overlay2/overlay.go

@@ -145,6 +145,14 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 		return nil, graphdriver.ErrNotSupported
 	}
 
+	fsMagic, err := graphdriver.GetFSMagic(testdir)
+	if err != nil {
+		return nil, err
+	}
+	if fsName, ok := graphdriver.FsNames[fsMagic]; ok {
+		backingFs = fsName
+	}
+
 	supportsDType, err := fsutils.SupportsDType(testdir)
 	if err != nil {
 		return nil, err