bonding: fix enslavement slave link notifications

Currently, link notifications are not sent by
bond_set_slave_link_state() upon enslavement if
the slave is enslaved when up.

This happens because slave->link default init value
is 0, which is the same as BOND_LINK_UP, resulting
in bond_set_slave_link_state() ignoring this transition.

This patch sets the default value of slave->link to
BOND_LINK_NOCHANGE, assuring it will count as a state
transition and thus trigger notification logic.

Signed-off-by: Aviv Heller <avivh@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>