kernel: fix bridge proxyarp issue with some broken DHCP clients

kernel: fix bridge proxyarp issue with some broken DHCP clients

There are broken devices in the wild that handle duplicate IP address
detection by sending out ARP requests for the IP that they received from a
DHCP server and refuse the address if they get a reply.
When proxyarp is enabled, they would go into a loop of requesting an address
and then NAKing it again.

Fixes: https://github.com/openwrt/openwrt/issues/14309
Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit c1ad78318c3e6421e60dd187477f38ca5f9a5752)