Implement new "learn" action.

There are a few loose ends here.  First, learning actions cause too much
flow revalidation.  Upcoming commits will fix that problem.  The following
additional issues have not yet been addressed:

    * Resource limits: nothing yet limits the maximum number of flows that
      can be learned.  It is possible to exhaust all system memory.

    * Age reporting: there is no way to find out how soon a learned table
      entry is due to be evicted.

To try this action out, here's a recipe for a very simple-minded MAC
learning switch.  It uses a 10-second MAC expiration time to make it easier
to see what's going on:

ovs-vsctl del-controller br0
ovs-ofctl del-flows br0
ovs-ofctl add-flow br0 "table=0 actions=learn(table=1, hard_timeout=10, \
	NXM_OF_VLAN_TCI[0..11], NXM_OF_ETH_DST[]=NXM_OF_ETH_SRC[], \
	output:NXM_OF_IN_PORT[]), resubmit(,1)"
ovs-ofctl add-flow br0 "table=1 priority=0 actions=flood"

You can then dump the MAC learning table with:

ovs-ofctl dump-flows br0 table=1

1 files changed, 0 insertions, 26 deletions

diff --git a/lib/meta-flow.c b/lib/meta-flow.c
index 9727253e..140f9022 100644
--- a/lib/meta-flow.c
+++ b/lib/meta-flow.c
@@ -302,32 +302,6 @@ static const struct mf_field mf_fields[MFF_N_IDS] = {
     }
 };
 
-static bool
-is_all_zeros(const uint8_t *field, size_t length)
-{
-    size_t i;
-
-    for (i = 0; i < length; i++) {
-        if (field[i] != 0x00) {
-            return false;
-        }
-    }
-    return true;
-}
-
-static bool
-is_all_ones(const uint8_t *field, size_t length)
-{
-    size_t i;
-
-    for (i = 0; i < length; i++) {
-        if (field[i] != 0xff) {
-            return false;
-        }
-    }
-    return true;
-}
-
 /* Returns the field with the given 'id'. */
 const struct mf_field *
 mf_from_id(enum mf_field_id id)