bonding: fix value exported by Netlink for peer_notif_delay

IFLA_BOND_PEER_NOTIF_DELAY was set to the value of downdelay instead
of peer_notif_delay. After this change, the correct value is exported.

Fixes: 07a4ddec

 ("bonding: add an option to specify a delay between peer notifications")
Signed-off-by: Vincent Bernat <vincent@bernat.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>