drbd: Actually allow BIOs up to 128k (was 32k).

Now we have multiple BIOs per ee, packets with a 32 bit length field,
it gets time to use these goodies.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>