containerd-shim: Do not remount root MS_SLAVE

Mounting as MS_SLAVE here breaks use cases which want to use
rootPropagation=shared in order to expose mounts to the host (and other
containers binding the same subtree), mounting as e.g. MS_SHARED is pointless
in this context so just remove.

Having done this we also need to arrange to manually clean up the mounts on
delete, so do so.

Note that runc will also setup root as required by rootPropagation, defaulting
to MS_PRIVATE.

Fixes #1132.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

mount/mount.go

@@ -22,3 +22,19 @@ func MountAll(mounts []Mount, target string) error {
 	}
 	return nil
 }
+
+// UnmountN tries to unmount the given mount point nr times, which is
+// useful for undoing a stack of mounts on the same mount
+// point. Returns the first error encountered, but always attempts the
+// full nr umounts.
+func UnmountN(mount string, flags, nr int) error {
+	var err error
+	for i := 0; i < nr; i++ {
+		if err2 := Unmount(mount, flags); err2 != nil {
+			if err == nil {
+				err = err2
+			}
+		}
+	}
+	return err
+}