i40e: Fix NULL ptr dereference on VSI filter sync

Remove the reason of null pointer dereference in sync VSI filters.
Added new I40E_VSI_RELEASING flag to signalize deleting and releasing
of VSI resources to sync this thread with sync filters subtask.
Without this patch it is possible to start update the VSI filter list
after VSI is removed, that's causing a kernel oops.

Fixes: 41c445ff ("i40e: main driver core")
Signed-off-by: Grzegorz Szczurek <grzegorzx.szczurek@intel.com>
Signed-off-by: Michal Maloszewski <michal.maloszewski@intel.com>
Reviewed-by: Przemyslaw Patynowski <przemyslawx.patynowski@intel.com>
Reviewed-by: Witold Fijalkowski <witoldx.fijalkowski@intel.com>
Reviewed-by: Jaroslaw Gawin <jaroslawx.gawin@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>