Add event-related mutex assertions

Add event-related mutex assertions

In utility functions that access fields of type struct rxevent *,
assert that the appropriate lock is held for the access in question.

These assertions are only compiled in when built with -DOPR_DEBUG_LOCKS,
which can be enbled by --debug-locks at configure time.

Reviewed-on: https://gerrit.openafs.org/12757
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit a7a3108e602c83176c5578c9f28b6312f71aba78)

Change-Id: I147a2e475feffb1b75a08ac5b08614bd6d8f46a5
Reviewed-on: https://gerrit.openafs.org/12776
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>