ubik: avoid DISK_Begin on sites that didn't vote for sync

ubik: avoid DISK_Begin on sites that didn't vote for sync

As already described on 7c708506, SDISK_Begin fails on remotes if
lastYesState is not set. To fix this problem, 7c708506 does not allow
write transactions until we know that lastYesState is set on at least
quorum (ubik_syncSiteAdvertised == 1). In other words, if enough sites
received a beacon packet informing that a sync-site was elected, write
transactions will be allowed. This means that ubik_syncSiteAdvertised
can be true while lastYesState is not set in a few sites.

Consider the following scenario in a cell with frequent write
transactions:

Site A => Sync-site (up)
Site B => Remote 1 (up)
Site C => Remote 2 (down - unreachable)

Since A and B are up, we have quorum. After the second wave of beacons,
ubik_syncSiteAdvertised will be true and write transactions will be
allowed. At some point, C is not unreachable anymore. Site A sends a
copy of its database to C, but C did not vote for A yet (lastYesState ==
0). A new write transaction is initialized and, since lastYesState is
not set on C, DISK_Begin fails on this remote site and C is marked as
down. Since C is reachable, A will mark this remote site as up. The
sync-site will send its database to C, but C did not vote for A yet. A
new write transaction is initialized and, since lastYesState is not set
on C, DISK_Begin fails on this remote site and C is marked as down. In a
cell with frequent write transactions, this cycle will repeat forever.
As a result, the sync-site will be constantly sending its database to C
and quorum will be operating with less sites, increasing the chances
of re-elections.

To fix this problem, do not call DISK_Begin on remotes that did not
vote for the sync-site yet.

Reviewed-on: https://gerrit.openafs.org/12715
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 68ec78950a6e39dc1bf15012d4b889728086d0b7)

Change-Id: I3764c23125f0bc675762449cd29b282ba403f871
Reviewed-on: https://gerrit.openafs.org/12896
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>