mac-learning: Only revalidate relevant flows upon flush.

mac-learning: Only revalidate relevant flows upon flush.

Most callers of mac_learning_flush() need to revalidate all flows
anyway, but this update forces new callers of mac_learning_flush()
to think about whether or not they need to explicitly handle
revalidation.  Also, it's theoretically more efficient when there
are lots of flows but the learning table is empty, (e.g. bridges
that don't use the NORMAL action).

Signed-off-by: Ethan Jackson <ethan@nicira.com>