Revert "vti4: Don't override MTU passed on link creation via IFLA_MTU"

This reverts commit 5815901c which is
03080e5ec727 ("vti4: Don't override MTU passed on link creation via
IFLA_MTU") upstream as it causes test failures.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>