Change default for DRBD barriers

Commit 8a69b3a8 changed the DRBD barriers disabled by default if
“--disable-drbd-barriers” is used to “bfd”, where “d” stands for
disk-drain. DRBD's upstream updated their documentation[1] to
say disk drains should always be used.

[1] http://git.drbd.org/?p=drbd-8.3.git;a=commit;h=f952dcfed57acf



Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>