hw/net/vmxnet3: allow VMXNET3_MAX_MTU itself as a value

hw/net/vmxnet3: allow VMXNET3_MAX_MTU itself as a value

Currently, VMXNET3_MAX_MTU itself (being 9000) is not considered a
valid value for the MTU, but a guest running ESXi 7.0 might try to
set it and fail the assert [0].

In the Linux kernel, dev->max_mtu itself is a valid value for the MTU
and for the vmxnet3 driver it's 9000, so a guest running Linux will
also fail the assert when trying to set an MTU of 9000.

VMXNET3_MAX_MTU and s->mtu don't seem to be used in relation to buffer
allocations/accesses, so allowing the upper limit itself as a value
should be fine.

[0]: https://forum.proxmox.com/threads/114011/

Fixes: d05dcd94ae ("net: vmxnet3: validate configuration values during activate (CVE-2021-20203)")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit 099a63828130843741d317cb28e936f468b2b53b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>