net: Do not turn on carrier when protodown is on

The protodown functionality allows user space to turn off the carrier of
a net device:

 # ip link add name dummy1 up type dummy
 # ip link add name macvlan1 up link dummy1 type macvlan mode bridge
 # ip link set dev macvlan1 protodown on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>

Different applications can set different protodown reasons, which
prevents an application from turning on the carrier of a net device as
long as others want it down:

 # ip link set dev macvlan1 protodown_reason 1 on
 # ip link set dev macvlan1 protodown_reason 2 on
 # ip link set dev macvlan1 protodown off
 Error: Cannot clear protodown, active reasons.
 # ip link set dev macvlan1 protodown_reason 2 off
 # ip link set dev macvlan1 protodown off
 Error: Cannot clear protodown, active reasons.
 # ip link set dev macvlan1 protodown_reason 1 off
 # ip link set dev macvlan1 protodown off
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  UP             0a:5c:a3:05:c7:86 <BROADCAST,MULTICAST,UP,LOWER_UP>

Unfortunately, this mechanism is not very useful when the carrier of a
net device can be toggled by toggling the carrier of its lower device:

 # ip link set dev macvlan1 protodown on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>
 # ip link set dev dummy1 carrier off
 # ip link set dev dummy1 carrier on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  UP             0a:5c:a3:05:c7:86 <BROADCAST,MULTICAST,UP,LOWER_UP>

Obviously, this is not the intended behavior and it is unlikely to be
relied on by anyone. In fact, it is a problem for applications like FRR
that use protodown with macvlan on top of a bridge as part of Virtual
Router Redundancy Protocol (VRRP).

Solve this by preventing a net device configured with protodown on from
gaining carrier by making netif_carrier_on() a NOP when protodown is
turned on.

Output with the patch:

 # ip link add name dummy1 up type dummy
 # ip link add name macvlan1 up link dummy1 type macvlan mode bridge
 # ip link set dev macvlan1 protodown on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>
 # ip link set dev dummy1 carrier off
 # ip link set dev dummy1 carrier on
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  DOWN           0a:5c:a3:05:c7:86 <NO-CARRIER,BROADCAST,MULTICAST,UP>
 # ip link set dev macvlan1 protodown off
 $ ip -br link show dev macvlan1
 macvlan1@dummy1  UP             0a:5c:a3:05:c7:86 <BROADCAST,MULTICAST,UP,LOWER_UP>

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260507105906.891817-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>