rx: Tidy up currentPacket handling

rx: Tidy up currentPacket handling

Instead of making a copy of the call->currentPacket variable in our
read/write routines, reference it directly. Make it clear that
currentPacket is used solely by the application thread, and remove
a number of mistaken comments that suggest otherwise.

Change-Id: I7ad799acbf110422df7c1e18ab552caf26b7766a
Reviewed-on: http://gerrit.openafs.org/7194
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>