igb: Fix use-after-free error during reset

Cleans the next descriptor to watch (next_to_watch) when cleaning the
TX ring.

Failure to do so can cause invalid memory accesses. If igb_poll() runs
while the controller is reset this can lead to the driver try to free
a skb that was already freed.

(The crash is harder to reproduce with the igb driver, but the same
potential problem exists as the code is identical to igc)

Fixes: 7cc6fd4c ("igb: Don't bother clearing Tx buffer_info in igb_clean_tx_ring")
Signed-off-by: NVinicius Costa Gomes <vinicius.gomes@intel.com>
Reported-by: NErez Geva <erez.geva.ext@siemens.com>
Tested-by: NTony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -4835,6 +4835,8 @@ static void igb_clean_tx_ring(struct igb_ring *tx_ring)
 					       DMA_TO_DEVICE);
 		}
 
+		tx_buffer->next_to_watch = NULL;
+
 		/* move us one more past the eop_desc for start of next pkt */
 		tx_buffer++;
 		i++;