i40evf: protect VLAN filter list

The MAC filter list is protected by a critical task bit, and the VLAN
list should be protected as well. This prevents list corruption if the
watchdog happens to run at the same time as a VLAN filter is being added
or deleted.

Change-ID: Ia4867cebbbb046a1f38012771b288a634ca5882b
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>