Both mega flow mask's reference counter and per flow table mask list
should only be accessed when holding ovs_mutex() lock. However
this is not true with ovs_flow_table_flush(). The patch fixes this bug.
Reported-by: NJoe Stringer <joestringer@nicira.com>
Signed-off-by: NAndy Zhou <azhou@nicira.com>
Signed-off-by: NJesse Gross <jesse@nicira.com>