DAFS: Do not record vol ops for DELETED vols

DAFS: Do not record vol ops for DELETED vols

When a volume is VOL_STATE_DELETED, it effectively does not exist, so
there is little point in recording a vp->pending_vol_op structure for
it. Just let callers checkout the volume as they would a nonexistent
volume: without recording anything about the operation.

This just reduces some edge cases and confusing debugging info, so we
don't have to worry about cleaning up pending_vol_op structures for
nonexistent volumes.

Change-Id: I56b92c3f2548ea48f9c61100be07edf9f7277fee
Reviewed-on: http://gerrit.openafs.org/4296
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>