libafscp: Actually return callback from FindCallback

Fix FindCallback so that it actually returns the callback that it
found. This requires changing the function prototype so that the
third parameter is passed by reference, and updating the single
call site.

Caught by clang-analyzer

Change-Id: I420647e9cfd119cf1a26456a91d595b8154ddd85
Reviewed-on: http://gerrit.openafs.org/9198
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>