e1000: Move cancel_work_sync to avoid deadlock

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

e1000: Move cancel_work_sync to avoid deadlock

Previously, e1000_down called cancel_work_sync for the e1000 reset task

(via e1000_down_and_stop), which takes RTNL.

As reported by users and syzbot, a deadlock is possible in the following

scenario:

CPU 0:

- RTNL is held

- e1000_close

- e1000_down

- cancel_work_sync (cancel / wait for e1000_reset_task())

CPU 1:

- process_one_work

- e1000_reset_task

- take RTNL

To remedy this, avoid calling cancel_work_sync from e1000_down

(e1000_reset_task does nothing if the device is down anyway). Instead,

call cancel_work_sync for e1000_reset_task when the device is being

removed.