ring_buffer: make finish methods final

Make it possible to "finish" with fewer bytes than what was "claimed".

This was possible before on the get side, but the put side was
cummulative wrt finish. The revamp made it cummulative on both sides.
Turns out that existing users rely on the opposite behavior which is
more logical and useful. So make both sides that way.

Adjust documentation, test case and users accordingly.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>