Derive mtu from ipv6 route to ipv4 /96 prefix instead of device

Test: builds, atest, still needs testing on real network
  x86 clatd_test
  --------------
  clatd_test (19 Tests)
  ...
  [6/19] ClatdTest#DetectMtu: PASSED (0ms)
  ...
Bug: 147935930
Signed-off-by: Maciej ┼╗enczykowski <maze@google.com>
Change-Id: I3d11ba082dabf70089867146acd84f6436953663
9 files changed