bonding: add slave_dev field for bond_opt_value

Currently, bond_opt_value are mostly used for bonding option settings. If
we want to set a value for slave, we need to re-alloc a string to store
both slave name and vlaue, like bond_option_queue_id_set() does, which
is complex and dumb.

As Jon suggested, let's add a union field slave_dev for bond_opt_value,
which will be benefit for future slave option setting. In function
__bond_opt_init(), we will always check the extra field and set it
if it's not NULL.

Suggested-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Jonathan Toppins <jtoppins@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>