bonding: reset bond's flags when down link is P2P device

When adding a point to point downlink to the bond, we neglected to reset
the bond's flags, which were still using flags like BROADCAST and
MULTICAST. Consequently, this would initiate ARP/DAD for P2P downlink
interfaces, such as when adding a GRE device to the bonding.

To address this issue, let's reset the bond's flags for P2P interfaces.

Before fix:
7: gre0@NONE: <POINTOPOINT,NOARP,SLAVE,UP,LOWER_UP> mtu 1500 qdisc noqueue master bond0 state UNKNOWN group default qlen 1000
    link/gre6 2006:70:10::1 peer 2006:70:10::2 permaddr 167f:18:f188::
8: bond0: <BROADCAST,MULTICAST,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/gre6 2006:70:10::1 brd 2006:70:10::2
    inet6 fe80::200:ff:fe00:0/64 scope link
       valid_lft forever preferred_lft forever

After fix:
7: gre0@NONE: <POINTOPOINT,NOARP,SLAVE,UP,LOWER_UP> mtu 1500 qdisc noqueue master bond2 state UNKNOWN group default qlen 1000
    link/gre6 2006:70:10::1 peer 2006:70:10::2 permaddr c29e:557a:e9d9::
8: bond0: <POINTOPOINT,NOARP,MASTER,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/gre6 2006:70:10::1 peer 2006:70:10::2
    inet6 fe80::1/64 scope link
       valid_lft forever preferred_lft forever

Reported-by: Liang Li <liali@redhat.com>
Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2221438


Fixes: 872254dd ("net/bonding: Enable bonding to enslave non ARPHRD_ETHER")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>