drbd: fix NULL pointer dereference on 4k hard sect size

we still don't support 4k 'physical' sectors 'natively',
but use a read-modify-write workaround.
And we even tried to use the extra page before we allocated it :(

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