vol: Add interfaces for registering RX calls

vol: Add interfaces for registering RX calls

Add VGetVolumeWithCall and VPutVolumeWithCall, to associate RX calls
with volume heavyweight references. Also add the interrupt_rxcall
field to the volume package options structure.

This also adds the VIsGoingOffline function, so a caller can tell when
a volume is going offline.

Change-Id: Iacb7738775c8e3aa611360320ca739f5de4ba625
Reviewed-on: http://gerrit.openafs.org/3215
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>