rx: Rework code which pulls packet from recv queue

rx: Rework code which pulls packet from recv queue

Both rxi_ReadProc and rxi_FillReadVector contained copies of the
same code to pull a packet out of the receive queue, and turn it
into the call's currentPacket. Abstract this out into a single common
function, so we're not maintaining the same code in two different
places.

Change-Id: I20af6b4ff19f05e21ffde1a80609be12ad6cfeee
Reviewed-on: http://gerrit.openafs.org/7195
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>