rx: rxi_ReceiveDataPacket do not set rprev on drop

rx: rxi_ReceiveDataPacket do not set rprev on drop

In KERNEL builds if there are no available packet buffers the
new packet is dropped on the floor.  In that case, the call's
rprev field should not be updated because the packet was never
"received" for delivery to the application.

Remove a dead comment from the same block of code.

Change-Id: I12a2b1618a430880d18efee48e6348e6f9ba8119
Reviewed-on: http://gerrit.openafs.org/7784
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>