Make MIN/MAX code in rx/rx_packet.h more readable

Eventually all MIN/MAX code in the tree should be handled uniformly,
but until that day, make this chunk of it more readable and
document the odd exception case for Linux kernel builds.

Change-Id: I4afe5d99b63010df831943e6b82ff89733b08066
Reviewed-on: http://gerrit.openafs.org/8871
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>