CVE-2021-47301 igb: Fix use-after-free error during reset

In the Linux kernel, the following vulnerability has been resolved:

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)