bonding: split alb_send_learning_packets()

Create alb_send_lp_vid(), which will handle the skb/lp creation, vlan
tagging and sending, and use it in alb_send_learning_packets().

This way all the logic remains in alb_send_learning_packets(), which
becomes a lot more cleaner and easier to understand.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>