opr: Add opr_containerof

opr: Add opr_containerof

Add the opr_containerof macro, which can be used to find the base
address of a structure which contains a member whose location is known.

This formulation is heavily used throughout OpenAFS to determine the
base address of structures containing queue pointers - this provides
a central definition, rather than coding it from scratch each time.

Change-Id: I2a32a214252ad304238f740ad92579e5bb7287be
Reviewed-on: http://gerrit.openafs.org/5836
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>