drbd: Bugfix for regression introduced with f9bc8913c06022e

If we intent to use the block_id member of an epoch entry,
we may not use the digest member.

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