bonding: add an option to specify a delay between peer notifications

Currently, gratuitous ARP/ND packets are sent every `miimon'
milliseconds. This commit allows a user to specify a custom delay
through a new option, `peer_notif_delay'.

Like for `updelay' and `downdelay', this delay should be a multiple of
`miimon' to avoid managing an additional work queue. The configuration
logic is copied from `updelay' and `downdelay'. However, the default
value cannot be set using a module parameter: Netlink or sysfs should
be used to configure this feature.

When setting `miimon' to 100 and `peer_notif_delay' to 500, we can
observe the 500 ms delay is respected:

    20:30:19.354693 ARP, Request who-has 203.0.113.10 tell 203.0.113.10, length 28
    20:30:19.874892 ARP, Request who-has 203.0.113.10 tell 203.0.113.10, length 28
    20:30:20.394919 ARP, Request who-has 203.0.113.10 tell 203.0.113.10, length 28
    20:30:20.914963 ARP, Request who-has 203.0.113.10 tell 203.0.113.10, length 28

In bond_mii_monitor(), I...