fix backingFs assignment

Signed-off-by: Jintao Zhang <zhangjintao9020@gmail.com>
(cherry picked from commit 18c22f5bc1a30d325ed7a14d118c8fcf20f2f509)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: fe2a25a7859167a5d03d87b7810a49007bee011b
Component: engine

Author: tao12345666333

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