afs: Do not invalidate all dcaches on startup

Commit 20b0c65a289e2b55fb6922c8f60e873f1f4c6f97 changed
afs_UFSGetDSlot to always treat a dslot entry as invalid if
'datavalid' was 0. This was to force the invalidation of the given
dslot if we were reading in a dslot from the free or discard list,
since the data in that dslot is not valid.

However, 'datavalid' is also 0 when we read in dcache entries from
disk on startup. So, this means that we invalidated all cache entries
when the client started up, effectively making our persistent cache
worthless.

Fix this by only forcing this invalidation when we are reading from a
free or discarded dcache, and not during the initial cache scan. That
is, when 'indexvalid' is 1, and 'datavalid' is 0.

The parameters for these Get*DSlot variants should maybe be changed to
be a little more clear, but for now, this is a targeted fix for this
specific issue.

FIXES 131655

Change-Id: Ic23f1e17cd966b8468849f54fc6fda37e1f61c0c
Reviewed-on: http://gerrit.openafs.org/9833
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>