lib/bfd: Used relaxed atomics and optimize bfd_should_process_flow().

lib/bfd: Used relaxed atomics and optimize bfd_should_process_flow().

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

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

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