ReplicatedPG: Fix bug in rollback

ReplicatedPG: Fix bug in rollback

Previously, _rollback_to assumed that the rollback was a noop if
ctx->clone_obc was set and it's prior version matches head's version.
However, this broke in sequences like:

Write "snap1 contents" to oid "blah"
create snapshot "snap1"
Write "snap2 contents" to oid "blah"
create snapshot "snap2"
rollback oid "blah" to snapshot "snap1"

In this case, make_writeable would have just cloned head to the snap2
clone, but the relevant clone is actually "snap1".  _rollback_to now
verifies that the most recent clone is the correct one before assuming
that head is already correct.

Signed-off-by: Samuel Just <samuelj@hq.newdream.net>