Looking around the net you can find values for the internal resistance of 9V batteries of around 1 to 2 Ohm. Which fits nicely to the picture of 6 1.5V cells in series, where you can find values of around 0.2 Ohm of internal resistance.
At 100mA there will be a voltage drop of 0.2V, shouldn't be much of an issue.
Generally batteries don't like spiky currents and a reasonably sized capacitor (1µF or something like that) near the LM317 input will probably help to reduce the strain on the battery a bit. The internal resistance and wire resistance together with the input capacitor will form a low pass filter which will smooth the spikes the battery will see.
Also the datasheet suggest that the output capacitor will improve transient responses, which might be beneficial for your application. (a motor gets switched on and off, or does it run always?)