vol: avoid query for parent id when deleting disk header

vol: avoid query for parent id when deleting disk header

When a DAFS volume server removes a volume disk header file (V*.vol),
the volume server invokes an fssync command to have the file server
delete the Volume Group Cache (VGC) entry corresponding to the volume id
and the parent id of the removed volume header.

The volume parent id is unknown to the volume server when removing a
volume disk header on behalf of a "vos zap -force" operation. In this
case, the volume server issues a fssync query to attempt look up to the
parent id from the file server's VGC.  If this fssync query fails for
some reason, volume server is unable to delete the VGC entry for the
deleted volume header. The volume server logs an error and vos zap
reports a undocumented error code.

One common way this can be encountered is to issue a "vos zap -force" on
a file server that has just been restarted. In this case, the VGC may
not be fully populated yet, so the volume server is not able to look up
the parent id of the given volume.

With this commit, relax the requirement for the parent id when deleting
VGC entries. A placeholder of 0 is used to mean any parent id for the
given volume id.

This obviates the need to query for the parent id when performing a "vos
zap -force", and allows the volume server to remove any VGC entries
associated with the volume id being zapped.

Reviewed-on: https://gerrit.openafs.org/12839
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 65b55bcc26f69f25c67518f672b34be73f3be370)

Change-Id: I2e927d7b388c7be36a67e196a3acb70e58c9a661
Reviewed-on: https://gerrit.openafs.org/13896
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Yadavendra Yadav <yadayada@in.ibm.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>