vti6: fix PMTU caching and reporting on xmit

When setting the skb->dst before doing the MTU check, the route PMTU
caching and reporting is done on the new dst which is about to be
released.

Instead, PMTU handling should be done using the original dst.

This is aligned with IPv4 VTI.

Fixes: ccd740cb ("vti6: Add pmtu handling to vti6_xmit.")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>