afs: Refactor GetDSlot parameters

afs: Refactor GetDSlot parameters

The 'indexvalid' and 'datavalid' parameters were really representing 3
different scenarios, not 2 different values with 2 possibilities each.
Change these to a single parameter, 'type', with 3 different values:
DSLOT_NEW, DSLOT_UNUSED, and DSLOT_VALID. Hopefully this will make the
relevant code paths easier to understand.

This should incur no functional change; it is just code
reorganization.

Change-Id: Iac921e74532de89121b461fa0f53ddb778735e0c
Reviewed-on: http://gerrit.openafs.org/9834
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>