Rx: only compute peer bytes sent and received if rx_stats_active

Rx: only compute peer bytes sent and received if rx_stats_active

Computing the bytes sent and received is an expensive operation.
If rx statistics collection has been disabled we should not collect
the peer data.  The most expensive operation is the rx_FindPeer()
call that is performed during rxi_ReadPacket().  rxi_ReadPacket()
is processed by the rx listener thread which must be as fast as
possible.

Change-Id: I5403c88aa85f9049fe50a9c1f3dbaad7d8b802bd
Reviewed-on: http://gerrit.openafs.org/2782
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>