vxlan: correct interface MTU when using hw offloads

Otherwise it breaks when offloading like checksum or TSO are used,
because second (encapsulated) ip_output() processing passes fragments of
the encapsulated packet down to the hardware interface.

Diagnosed by: hselasky
Reviewed by: np
Sponsored by: Nvidia Networking / Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D29501