Support overlay2 on btrfs

OverlayFS is supported on top of btrfs as of Linux Kernel 4.7.
Skip the hard enforcement when on kernel 4.7 or newer and
respect the kernel check override flag on older kernels.
https://btrfs.wiki.kernel.org/index.php/Changelog#By_feature

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

daemon/graphdriver/overlay2/overlay.go

@@ -150,9 +150,19 @@ func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (grap
 
 	// check if they are running over btrfs, aufs, zfs, overlay, or ecryptfs
 	switch fsMagic {
-	case graphdriver.FsMagicBtrfs, graphdriver.FsMagicAufs, graphdriver.FsMagicZfs, graphdriver.FsMagicOverlay, graphdriver.FsMagicEcryptfs:
+	case graphdriver.FsMagicAufs, graphdriver.FsMagicZfs, graphdriver.FsMagicOverlay, graphdriver.FsMagicEcryptfs:
 		logrus.Errorf("'overlay2' is not supported over %s", backingFs)
 		return nil, graphdriver.ErrIncompatibleFS
+	case graphdriver.FsMagicBtrfs:
+		// Support for OverlayFS on BTRFS was added in kernel 4.7
+		// See https://btrfs.wiki.kernel.org/index.php/Changelog
+		if kernel.CompareKernelVersion(*v, kernel.VersionInfo{Kernel: 4, Major: 7, Minor: 0}) < 0 {
+			if !opts.overrideKernelCheck {
+				logrus.Errorf("'overlay2' requires kernel 4.7 to use on %s", backingFs)
+				return nil, graphdriver.ErrIncompatibleFS
+			}
+			logrus.Warn("Using pre-4.7.0 kernel for overlay2 on btrfs, may require kernel update")
+		}
 	}
 
 	rootUID, rootGID, err := idtools.GetRootUIDGID(uidMaps, gidMaps)