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.

Change-Id: Ia69e25fa73e10dc10cf3ddf08bb4feb2c9958674
Reviewed-on: http://gerrit.openafs.org/5526
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>