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: NGrzegorz Szczurek <grzegorzx.szczurek@intel.com>
Signed-off-by: NMichal Maloszewski <michal.maloszewski@intel.com>
Reviewed-by: NPrzemyslaw Patynowski <przemyslawx.patynowski@intel.com>
Reviewed-by: NWitold Fijalkowski <witoldx.fijalkowski@intel.com>
Reviewed-by: NJaroslaw Gawin <jaroslawx.gawin@intel.com>
Reviewed-by: NAleksandr Loktionov <aleksandr.loktionov@intel.com>
Tested-by: NTony Brelinski <tony.brelinski@intel.com>
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>