Using ifs is definitely more readable than &&, words always win. However carefully alignment of && and ( can make things better. Note that the comment after "if" is necessary to keep the shape balanced.

    if( /* b is same as a */
           (b != nil)
        && (b->qid.type == a->qid.type)
        && (b->qid.path == a->qid.path)
        && (b->qid.vers == a->qid.vers)
        && (b->dev == a->dev)
        && (b->type == a->type)
    ){
        fprint(2, "cp: %s and %s are the same file\n", an, bn);
        ret = 1;
    }