Enable LRO by default when available on vtnet interfaces

The prior change to not enable LRO by default has confused several
people. The configurations where LRO is problematic is not the
typical use case for VirtIO, and due to other issues, this often
requires checksum offloading to be disabled anyways.

PR:		185864
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274325 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f