ofproto/bond: Minor cleanups.

Add a constant for the number of bond buckets, and other minor cleanups.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>¬
ofproto/bond: Protect statistics with writelock.

dcf00ba35a0 (ofproto/bond: Implement bond megaflow using recirculation)
allowed bond_entry statistics to be modified while holding a readlock.
This patch modifies bond_entry_account() to get a writelock before
modifying the statistics and adds thread-safety annotations to these
fields and relevant functions.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>¬