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

[kib]

Summary

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.

Sponsored by: Mellanox Technologies/NVidia Networking

[kib]

Committed as 994e47023ac87ede16e9c785ac5d6880e3d3a23c