net/bonding: Implement ndo_sk_get_lower_dev

Add ndo_sk_get_lower_dev() implementation for bond interfaces.

Support only for the cases where the socket's and SKBs' hash
yields identical value for the whole connection lifetime.

Here we restrict it to L3+4 sockets only, with
xmit_hash_policy==LAYER34 and bond modes xor/802.3ad.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Boris Pismenny <borisp@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/bonding.h

@@ -265,6 +265,8 @@ struct bond_vlan_tag {
 	unsigned short	vlan_id;
 };
 
+bool bond_sk_check(struct bonding *bond);
+
 /**
  * Returns NULL if the net_device does not belong to any of the bond's slaves
  *