vtnet: deprecate loader tunable fixup_needs_csum

If this tunable is enabled and vtnet receives a packet with
VIRTIO_NET_HDR_F_NEEDS_CSUM set, vtnet computes the TCP/UDP checksum
and writes it in the checksum field.
This was somewhat useful when vtnet pretended that such a packet has
a correct checksum and set the mbuf flag CSUM_DATA_VALID.
But this is not the case anymore.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D52546

(cherry picked from commit 5da388d93917f5fa74022960cc65452592f71539)