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.

This commit should not have been backported to anything older than 4.16,
despite what the changelog said as the mtu must be set in older kernels,
unlike is needed in 4.16 and newer.

Thanks to Alistair Strachan for the debugging help figuring this out,
and for 'git bisect' for making my life a whole lot easier.

Cc: Alistair Strachan <astrachan@google.com>
Cc: Stefano Brivio <sbrivio@redhat.com>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>