ofproto-bond: do not allow recirculation when we failed to allocate recirc_id

When recirc pool is exhausted, a new bond won't be allocate a new
recirc_id. The bond->recirc_id will remain zero. This condition
should prevent the bond from use recirculation. This check was missing
before this patch.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
ofproto-dpif: restore bond rebalance for non-recirc bond

Bond rebalancing was disabled for bonds not using recirculation. The
patch fixes this bug.

While fixing the bug, the bond_rebalance() was also restructured
slightly to move bond related logic back into ofproto/bond.c

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>