snapshot: use temporary lease during diffapply.

A temporary lease ctx was previously made during the snapshotter Merge
method but it was not passed to diffApply, which may have meant that
temporary View snapshots created there could be garbage collected by
containerd at any time.

It's possible this was encountered in CI, but I was unable to reproduce
the failure after 1000 attempts locally, so this commit is just a best
guess at the problem.

Signed-off-by: Erik Sipsma <[email protected]>
(cherry picked from commit 868f78d)
Signed-off-by: Tonis Tiigi <[email protected]>

snapshot/merge.go

@@ -130,15 +130,15 @@ func (sn *mergeSnapshotter) Merge(ctx context.Context, key string, diffs []Diff,
 		diffs = diffs[baseIndex:]
 	}
 
-	tempLeaseCtx, done, err := leaseutil.WithLease(ctx, sn.lm, leaseutil.MakeTemporary)
+	ctx, done, err := leaseutil.WithLease(ctx, sn.lm, leaseutil.MakeTemporary)
 	if err != nil {
 		return errors.Wrap(err, "failed to create temporary lease for view mounts during merge")
 	}
 	defer done(context.TODO())
 
 	// Make the snapshot that will be merged into
 	prepareKey := identity.NewID()
-	if err := sn.Prepare(tempLeaseCtx, prepareKey, baseKey); err != nil {
+	if err := sn.Prepare(ctx, prepareKey, baseKey); err != nil {
 		return errors.Wrapf(err, "failed to prepare %q", key)
 	}
 	applyMounts, err := sn.Mounts(ctx, prepareKey)