LU-11719 ldlm: Adjust search_* functions

LU-11719 ldlm: Adjust search_* functions

The search_itree and search_queue functions should both
return either a pointer to a found lock or NULL.

Currently, search_itree just returns the contents of
data->lmd_lock, whether or not a lock was found.

search_queue will do the same under certain cirumstances.

Zero lmd_lock in both search_* functions, and also stop
searching in search_itree once a lock is found.

cray-bug-id: LUS-6783
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: Ie231166756e60c228370f8f1a019ccfe14dfda6a
Reviewed-on: https://review.whamcloud.com/33754
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>