i40e: check __I40E_VF_DISABLE bit in i40e_sync_filters_subtask

While testing VF spawn/destroy the following panic occurred.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000029
[...]
Workqueue: i40e i40e_service_task [i40e]
RIP: 0010:i40e_sync_vsi_filters+0x6fd/0xc60 [i40e]
[...]
Call Trace:
 ? __switch_to_asm+0x35/0x70
 ? __switch_to_asm+0x41/0x70
 ? __switch_to_asm+0x35/0x70
 ? _cond_resched+0x15/0x30
 i40e_sync_filters_subtask+0x56/0x70 [i40e]
 i40e_service_task+0x382/0x11b0 [i40e]
 ? __switch_to_asm+0x41/0x70
 ? __switch_to_asm+0x41/0x70
 process_one_work+0x1a7/0x3b0
 worker_thread+0x30/0x390
 ? create_worker+0x1a0/0x1a0
 kthread+0x112/0x130
 ? kthread_bind+0x30/0x30
 ret_from_fork+0x35/0x40

Investigation revealed a race where pf->vf[vsi->vf_id].trusted may get
accessed by the watchdog via i40e_sync_filters_subtask() although
i40e_free_vfs() already free'd pf->vf.
To avoid this the call to i40e_sync_vsi_filters() in
i40e_sync_filters_subtask() needs to be guarded by __I40E_VF_DISABLE,
which is also used by i40e_free_vfs().

Note: put the __I40E_VF_DISABLE check after the
__I40E_MACVLAN_SYNC_PENDING check as the latter is more likely to
trigger.

CC: stable@vger.kernel.org
Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>