L2 MTU includes both payload and ethernet header. gVisor has supported setting
data link layer MTU, it will be up to users to set up proper value for a
device's MTU if a larger value is expected.
By doing the change, gVisor will be consistent with runc:
```shell
# runc
root@cec4d7238802:/# ip link set dev lo mtu 1234
root@cec4d7238802:/# ip link list dev lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 1234 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
# runsc before the change.
root@7cd89278d414:/# ip link set dev lo mtu 1234
root@7cd89278d414:/# ip link list dev lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 1220
link/loopback 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
# runsc after the change.
root@e811c5851226:/# ip link set dev lo mtu 1234
root@e811c5851226:/# ip link list dev lo
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 1234
link/loopback 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
```
PiperOrigin-RevId: 684746090
The error code is based on the runc's behavior:
```shell
# ip link set dev br0 master br0
Error: Can not enslave a bridge to a bridge.
# echo $?
2
```
Reported-by: syzbot+9dcefbdcc9ecdeb4df96@syzkaller.appspotmail.com
PiperOrigin-RevId: 679761515
IFLA_IFNAME is used to set an interface's name when being used
with RTM_NEWLINK and RTM_SETLINK. RTM_NEWLINK is the preferred way, it
is backwards compatible with RTM_SETLINK. For gVisor, when RTM_SETLINK is
introduced, it shall just share the implementation.
PiperOrigin-RevId: 647833657
Jing Chen
Andrei Vagin