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.

Reviewed-on: http://gerrit.openafs.org/2997
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 525ea8b60cc198f7e12da7a1aaffdcad2bb3ef21)
Change-Id: Ide4c225516dc7585b49ba7b244b56446297c74d1
Reviewed-on: http://gerrit.openafs.org/3112