Recover from afs_GetVolSlot errors

Recover from afs_GetVolSlot errors

afs_GetVolSlot can panic in a few different ways, such as failing to
read from or write to VolumeInfo. Instead of panic'ing, return an
error to the application. Adjust callers to deal with getting a NULL
volume returned.

Based on a patch by Mike Meffie.

Change-Id: Ibb301fed795ee9ee5906b8e7973945a06218b8f1
Reviewed-on: http://gerrit.openafs.org/1801
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry-picked from commit 80f05473428969ff71d1791cda8cb09d30223724)

Change-Id: Ie89034cb0fcc8f92868f338996d62edfe150b1b2
Reviewed-on: http://gerrit.openafs.org/2004
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>