dlm: use RSB_HASHED to avoid lookup twice

Since commit 01fdeca1 ("dlm: use rcu to avoid an extra rsb struct
lookup") _dlm_master_lookup() is called under rcu lock that prevents
that the rsb structure is being freed. There was a missing change to
avoid an additional lookup and just check that the rsb is still part of
the ls_rsbtbl structure. This patch is doing such check instead of
lookup the rsb structure again.

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