dlm: fix uninitialized variable for search_rsb_list callers

gcc 4.3.0 correctly emits the following warning.
search_rsb_list does not *r_ret if no dlm_rsb is found
and _search_rsb may pass the uninitialized value upstream
on the error path when both calls to search_rsb_list
return non-zero error.

The fix sets *r_ret to NULL on search_rsb_list's not-found path.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: David Teigland <teigland@redhat.com>