gve: Disallow MTUs within a problematic range

If hardware LRO is enabled with GVE, then setting the driver's MTU to a
range of values around 8000 will cause dropped packets and drastically
degraded performance. While this issue is being investigated, we need
to prohibit the driver's MTU being set to a value within this range.

Signed-off-by: Jasper Tran O'Leary <jtranoleary@google.com>

Reviewed by: delphij, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D48971