Obtain the MTU from a socket to the destination

The MTU from the socket is a much easier way to get the
correct MTU to any given destination. In particular this
is important for CLAT which used to be wrong :
on my device, with a perfectly working GoogleGuest :
50: v4-wlan0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1472 qdisc pfifo_fast state UNKNOWN group default qlen 500

  F  ICMPv4 payloadLength{1472} dst{8.8.8.8} src{192.0.0.4:27}: FAILED: 0/15 (4811ms)

After this patch, the payload is correctly 1444.

Test: Manual on GoogleGuest
      Manual on a network with a custom set MTU
Change-Id: I6cd2398572b3fe08f88832ab6d0aa715c118cd4f