Make preparePost more readable

Co-Authored-By: Whyrusleeping <why@ipfs.io>

storage/post.go

@@ -149,7 +149,9 @@ func (p *post) preparePost(ctx context.Context) error {
 	}
 	p.sset = sset
 
-	r, err := p.m.api.ChainGetRandomness(ctx, p.ts, nil, int(int64(p.ts.Height())-int64(p.ppe)+int64(build.PoStChallangeTime)+int64(build.PoStRandomnessLookback))) // TODO: review: check math
+        // Compute how many blocks back we have to look from the given tipset for the PoSt challenge
+        challengeLookback := int((int64(p.ts.Height())-int64(p.ppe))+int64(build.PoStChallangeTime)+int64(build.PoStRandomnessLookback))
+	r, err := p.m.api.ChainGetRandomness(ctx, p.ts, nil, challengeLookback)
 	if err != nil {
 		return xerrors.Errorf("failed to get chain randomness for post (ts=%d; ppe=%d): %w", p.ts.Height(), p.ppe, err)
 	}