c8d: mark stargz as requiring reference-counted mounts

The stargz snapshotter cannot be re-mounted, so the reference-counted
path must be used.

Co-authored-by: Djordje Lukic <djordje.lukic@docker.com>
Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>
(cherry picked from commit 21c0a54a6b2dc059fae62f46a23d338a35fe6d1d)
Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

daemon/snapshotter/mount.go

@@ -14,7 +14,7 @@ import (
 const mountsDir = "rootfs"
 
 // List of known filesystems that can't be re-mounted or have shared layers
-var refCountedFileSystems = []string{"overlayfs", "zfs", "fuse-overlayfs"}
+var refCountedFileSystems = []string{"fuse-overlayfs", "overlayfs", "stargz", "zfs"}
 
 // Mounter handles mounting/unmounting things coming in from a snapshotter
 // with optional reference counting if needed by the filesystem