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

Reviewed-on: http://gerrit.openafs.org/9198
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit f67e4a3cdfb1856e31dda0d4741c8d34df13f8fc)

Change-Id: Ia610993535acc68beef7e8cffcfac77db2638aa4
Reviewed-on: http://gerrit.openafs.org/9973
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>