viced: Allow null host for BreakCallBack

viced: Allow null host for BreakCallBack

For replication writes at the remote site, we will want to call
this without a host structure.

Reviewed-on: http://gerrit.openafs.org/6674
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 01301d0a5323a836efaae30cac325c25f6a7577a)

Change-Id: I1fb0dff655515fedd7dfb41139f1fb6c85599377
Reviewed-on: http://gerrit.openafs.org/6753
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>