datapath: remove RCU annotation from flow->mask

After a mask is assigned to a flow, it will not change for the life of
the flow. Since flow access is protected by RCU lock, access to
flow->mask after getting a flow is always safe.

Suggested-by: Jesse Gross <jesse@nicira.com>
Reported-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>