recirculation: RCU postpone the free of dpif_backer_recirc_node.

This commit RCU postpones the free of 'struct dpif_backer_recirc_node',
after it is removed from the cmap.  This is in that other threads may
be accessing the struct at the same time.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>