Fix afs_GetVolume() for non-root dynroot FIDs

Fix afs_GetVolume() for non-root dynroot FIDs

Make afs_GetVolume() correctly handle requests for fids which are in
the dynroot cell but are not the root of the dynamic root volume.  This
is necessary to allow dynamic root mount points to be looked up and
followed in situations where the dynroot volume is not in the volcache,
but its root vnode is in the vcache.

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/180
Tested-by: Jeffrey Hutzelman <jhutz@cmu.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>