dlm: avoid unnecessary search in search_rsb

If the rsb is found in the "keep" tree, but is
not the right type (i.e. not MASTER), we can
return immediately with the result.  There's
no point in going on to search the "toss" list
as if we hadn't found it.

Signed-off-by: David Teigland <teigland@redhat.com>