bonding: trivial: rename slave->jiffies to ->last_link_up

slave->jiffies is updated every time the slave becomes active, which, for
bonding, means that its link is 'up'.

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>