Join to AllHosts multicast group again when adding an existing IPv4 address.

When SIOCAIFADDR ioctl configures an IPv4 address that is already exist,
it removes old ifaddr. When this IPv4 address is only one configured on
the interface, this also leads to leaving from AllHosts multicast group.
Then an address is added again, but due to the bug, this doesn't lead
to joining to AllHosts multicast group.

Submitted by: yannis.planus_alstomgroup.com
Reviewed by: gnn
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26757