vxlan: stop checking CSUM_ENCAP_VXLAN when converting inner CSUM flags into normal, for decapsulation.

The packet, if processed at this point, was already parsed to be UDP
directed to a vxlan port.

Connect-X 4+ does not provide easy method to infer which parser
processed the packet, so driver cannot set the flag without a lot of
efforts which are only to satisfy the formal requirements.

Reviewed by: bryanv, np
Sponsored by: Mellanox Technologies/NVidia Networking
Differential revision: https://reviews.freebsd.org/D27449
MFC after: 1 week