vxlan: fix incorrect nlattr access in MTU check

The access to the wrong variable could lead to a NULL dereference and
possibly other invalid memory reads in vxlan newlink/changelink requests
with a IFLA_MTU attribute.

Fixes: a985343b

 "vxlan: refactor verification and application of configuration"
Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: David S. Miller <davem@davemloft.net>