rx: Turn the rxevent_Cancel macro into a function

rx: Turn the rxevent_Cancel macro into a function

Turn rxevent_Cancel into a function rather than a macro which modifies
its argument as a side effect. rxevent_Cancel now checks whether the
event being cancelled is already NULL, as well as NULLifying the event
when it is actually cancelled.

Update all of the callers to reflect this new API, and so they no
longer do unecessary work.

Change-Id: I75b68f1c8f1a3023edd6113600663fe2b60d6097
Reviewed-on: http://gerrit.openafs.org/5840
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>