Support for SIOCSIFMTU socket ioctl

Linux kernel supports setting the MTU (Maximum Transfer Unit) of a device using socket ioctl SIOCSIFMTU.
see https://man7.org/linux/man-pages/man7/netdevice.7.html for details.

Currently gVisor _partially_ handles the following requests:
```
SIOCGIFNAME = 0x8910
SIOCGIFCONF = 0x8912
SIOCGIFFLAGS = 0x8913
SIOCGIFADDR = 0x8915
SIOCGIFDSTADDR = 0x8917
SIOCGIFBRDADDR = 0x8919
SIOCGIFNETMASK = 0x891b
SIOCGIFMETRIC = 0x891d
SIOCGIFMTU = 0x8921
SIOCGIFMEM = 0x891f
SIOCGIFHWADDR = 0x8927
SIOCGIFINDEX = 0x8933
SIOCGIFPFLAGS = 0x8935
SIOCGIFTXQLEN = 0x8942
SIOCETHTOOL = 0x8946
SIOCGMIIPHY = 0x8947
SIOCGMIIREG = 0x8948
SIOCGIFMAP = 0x8970
```

I currently focus on the SIOCSIFMTU for a personal project, but it is a good place to point out that there are many socket ioctls implemented by the Linux kernel which are not yet supported by gVisor.