RDMA/nes: Fix slab corruption

Referencing cm_node after it is freed via rem_ref_cm_node() causes a
slab corruption.  There is no need to set cm_node->cm_id to NULL in
mini_cm_close().

Signed-off-by: Chien Tung <ctung@neteffect.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>