vol: Do not give back not-checked-out vols

vol: Do not give back not-checked-out vols

VAttachVolumeByName_r has logic to give back a volume over FSSYNC if
we checked out a volume but failed to attach it for whatever reason.
However, the logic used for determining if the volume was checked out
or not is a bit inaccurate (even moreso than the comments imply),
potentially causing us to VOL_ON volumes that don't exist at all.

Instead of trying to guess based on various conditions whether or not
we checked out the volume, keep track of a variable that is only set
when we actually checkout the volume from the fileserver. Then only
give back the volume if it is set.

Change-Id: I03197eca3e1a31a4b9566552eb9032fdc7cc5909
Reviewed-on: http://gerrit.openafs.org/3274
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>