Rx: Do not compute RTT on non-last packets of a jumbogram

Rx: Do not compute RTT on non-last packets of a jumbogram

A jumbogram is constructed as a series of rx packets that are
all sent at once and acknowledged at the same time.  Computing the
RTT for all of the packets that makes up the jumbogram provides
the jumbogram RTT more weight than for a non-jumbogram packet.
To restore fairness, only compute the RTT for the last packet of
a jumbogram.  The non-last packets with have the RX_JUMBO_PACKET flag
set in the packet header.

Change-Id: Ia5237d9ca0707b39fe623639233b748b6798063e
Reviewed-on: http://gerrit.openafs.org/2997
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>