rx: Refactor rxi_ReceivePacket call selection

rx: Refactor rxi_ReceivePacket call selection

Refactor the call selection logic in rxi_ReceivePacket so that it is
a little bit easier to follow, and better optimised to the common case.

Split the current logic into a function for packets being received by
a server, and a function for packets being received by a client.

Change-Id: Ie27de7952cc13fa3b92619cfe68e671e6d5e170c
Reviewed-on: http://gerrit.openafs.org/8297
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>