vnic: apply hardware L3 checksum only for IPv4

Previously we set the csum_l3 flag for IPv4 and IPv6, but only IPv4
should have header checksumming applied.

Prompted by Linux commit fa6d7cb5d76cf0467c61420fc9238045aedfd379.

Reviewed by: bz
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13366