You will hit the walls because of the rotation of the planet. If the hole is straight through the poles, you will hit the walls because the planet is not gravametrically perfect, and distortions (clumps of mass inside the planet) will move you against the walls. If it was through the poles and the planet had no gravity clumps, *and* you were able to accurate steer yourself like a skydiver, air friction would slow you down until you have wound up in the middle of the core, floating there indefinitely. If there was no air friction, no clumps, and through the poles (or through a non-rotating planet), you would indefinitely go in one side and out the other, forever.