Fix: crmd: cl#5152 - Prevent a fenced node from being cleaned up after rejoining

If tengine_stonith_notify() is invoked during the state transition:
    S_ELECTION -> S_INTEGRATION -> S_FINALIZE_JOIN
we may have just missed A_DC_TAKEOVER, in which it does fencing cleanup.
Since tengine_stonith_callback() calls send_stonith_update(),
if we still store the node into stonith_cleanup_list,
the node will be cleaned up again after the next DC election in the furture by mistake.
So we do _not_ add the node into stonith_cleanup_list in this case:
Even if fsa_our_dc is NULL, but AM_I_DC is already TRUE.