dlm: move dlm_search_rsb_tree() out of lock

The rhashtable structure is lockless for readers such as
rhashtable_lookup_fast(). It should be save to call this lookup
functionality out of holding ls_rsbtbl_lock to get the rsb pointer out
of the hash. This reduce the contention time of ls_rsbtbl_lock in some
cases. We still need to check if the rsb is part of the check as this
state can be changed while ls_rsbtbl_lock is not held. If its part of
the rhashtable data structure we take a reference to be sure it will not
be freed after we drop the ls_rsbtbl_lock read lock.

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