rx: arrange for Finalize to really stop running calls

rx: arrange for Finalize to really stop running calls

previously rxi_ServerProc would happily error a call once
rx_tranquil was set, but keep calling ExecuteRequest.
Reorder code so kernel shutdown attempts are processed first;
then arrange if we are tranquil to not process the call further.

Issue discovered by Chaskiel Grundman.

Change-Id: I545960fb0eb0d5151baa7e06e2758cdeaf0885da
Reviewed-on: http://gerrit.openafs.org/5447
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>