Tethering: allow ipv6 1500 mtu again

but still fallback to 1400 if we don't know any better

Bug: 432676104
Test: TreeHugger, mm in p/m/C
Flag: EXEMPT mainline
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I7e1d70e73d2fd24d1f79dc40f3b18a6d82d235ef
1 file changed