vtnet: Support jumbo frames without TSO/GSO

Currently in Virtio driver without TSO/GSO features enabled, the max scatter
gather segments for the TX path can be 4, which limits the support for 9K JUMBO
frames. 9K JUMBO frames results in more than 4 scatter gather segments and
virtio driver fails to send the frame down to host OS. With TSO/GSO feature
enabled max scatter gather segments can be 64, then 9K JUMBO frames are fine,
this is making virtio driver to support JUMBO frames only with TSO/GSO.

Increasing the VTNET_MIN_TX_SEGS which is the case for non TSO/GSO to 32 to
support upto 64K JUMBO frames to Host.

Submitted by: Lohith Bellad <lohithbsd@gmail.com>
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D8803