rx: Tidy up variables in RTT calc

rx: Tidy up variables in RTT calc

We used to do rttp = &thisRtt, and then use rttp and thisRtt to
interchangably refer to the same data. This is just confusing, and
unnecessary. Replace all of the occurences of rttp with &thisRtt.

Take the opportunity to use the Clock_IsZero macro rather than doing
an explicit zero clock check.

Reviewed-on: http://gerrit.openafs.org/3031
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 58e7b85ab7c3c839c5dd126bd1c254f68a16b73a)

Change-Id: If2ad45645bc27af3a8ecd97f047b5558c73b173d
Reviewed-on: http://gerrit.openafs.org/3144
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>