lib/cfm: Use relaxed atomics and optimize cfm_should_process_flow().

lib/cfm: Use relaxed atomics and optimize cfm_should_process_flow().

The atomics here do not synchronize the state of any other variables,
so we can use relaxed atomics.

cfm_should_process_flow() is rearranged to set the megaflow mask bits
only if necessary, and to avoid the atomic operation on non-CFM
packets.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>