virtchnl: fix fake 1-elem arrays for structures allocated as `nents`

Finally, fix 3 structures which are allocated technically correctly,
i.e. the calculated size equals to the one that struct_size() would
return, except for sizeof(). For &virtchnl_vlan_filter_list_v2, use
the same approach when there are no enough space as taken previously
for &virtchnl_vlan_filter_list, i.e. let the maximum size be calculated
automatically instead of trying to guestimate it using maths.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>