if_vtnet: improve dumping a kernel

Disable software LRO during kernel dumping, because having it enabled
requires to be in a network epoch, which might or might not be the
case depending on the code path resulting in the panic.

Reviewed by: markj
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D34787

(cherry picked from commit 53236f905dbf4f3d58b1d5eff5c9b14f100d0c5d)