Revert "ip6_vti: adjust vti mtu according to mtu of lower device"

This reverts commit 1139d77d which is
commit 53c81e95df1793933f87748d36070a721f6cb287 upstream.

Ben writes that there are a number of follow-on patches needed to fix
this up, but they get complex to backport, and some custom fixes are
needed, so let's just revert this and wait for a "real" set of patches
to resolve this to be submitted if it is really needed.

Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Cc: Petr Vorel <pvorel@suse.cz>
Cc: Alexey Kodanev <alexey.kodanev@oracle.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>