DAFS: Add explicit 'valid' field for index maps

DAFS: Add explicit 'valid' field for index maps

The CB, FE, and host serialization structures were just using the
relevant indices to determine whether or not an entry mapping and old
index to a new index was populated with actual data. For host
structures, this really isn't sufficient, since our index can be 0,
and the structure is calloc'd, so the index in the structure could
also be 0.

Add a flag explicitly stating whether or not the structure has been
filled in, to make this unambiguous.

Reviewed-on: http://gerrit.openafs.org/5526
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit d54a9994d362ce3f287fe786839ec72f6d94806c)

Change-Id: I0bd3e50dec4e686acc3e9cda3eef7b7909266f27
Reviewed-on: http://gerrit.openafs.org/5544
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>