GetVolume: do not wait for offlining volumes

GetVolume: do not wait for offlining volumes

In non-DAFS GetVolume, previously we waited for a volume with
vp->goingOffline set to go offline before we return. However, the
fileserver afsfileprocs.c case can treat the "going offline" case and
the "is offline" case identically, so there is no reason to wait. So
make the fileserver call a new GetVolume variant that does not wait.

Without this, all of the threads in the fileserver can potentially be
tied up by waiting for a volume to go offline, if the volume is taking
a long time to go offline due to an offline request following a client
slowly accessing the volume.

Change-Id: I58ae11e585852130154389d8df0567432cd0c2df
Reviewed-on: http://gerrit.openafs.org/2124
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Tom Keiser <tkeiser@sinenomine.net>
Tested-by: Derrick Brashear <shadow@dementia.org>