vtnet: disable hardware TCP LRO by default

Hardware TCP LRO results in problems in settings with IP forwarding
being enabled. In case of nodes without IP forwarding, using
software LRO is also beneficial in general, since it can provide better
information about what was received on the wire.
Therefore, disable hardware TCP LRO by default.
By tuning the loader tunable, this can be changed.

PR: 263229
Reviewed by: Timo Völker
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52684