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: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Reported-by: Erez Geva <erez.geva.ext@siemens.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>