buffer: only check if instance is Uint8Array

Native Buffer method calls do not require anything from the prototype.
So it is unnecessary to check if the Object's prototype is equal to
Buffer.prototype.

This fixes an issue that prevents Buffer from being inherited the ES5
way. Now the following will work:

    function A(n) {
      const b = new Buffer(n);
      Object.setPrototypeOf(b, A.prototype);
      return b;
    }

    Object.setPrototypeOf(A.prototype, Buffer.prototype);
    Object.setPrototypeOf(A, Buffer);

    console.log(new A(4));

Fix: https://github.com/nodejs/node/issues/2882
PR-URL: https://github.com/nodejs/node/pull/3080


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>