bridge/link: add learning_sync policy flag

bridge/link: add learning_sync policy flag

v2:

Resending now that the dust has cleared in 3.18 on "self" vs. hwmode debate for
brport settings.  learning_sync is now set/cleared using "self" qualifier on
brport.

v1:

Add 'learned_sync' flag to turn on/off syncing of learned MAC addresses from
offload device to bridge's FDB.   Flag is be set/cleared on offload device port
using "self" qualifier:

  $ sudo bridge link set dev swp1 learning_sync on self

  $ bridge -d link show dev swp1
  2: swp1 state UNKNOWN : <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 master br0 state forwarding priority 32 cost 2
      hairpin off guard off root_block off fastleave off learning off flood off
  2: swp1 state UNKNOWN : <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 master br0
      learning on learning_sync on

Adds new IFLA_BRPORT_LEARNED_SYNCED attribute for IFLA_PROTINFO on the SELF
brport.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>