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.

Change-Id: I16885a4d37a0f094f0d365c54e8157ed92070c69
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>