Seal transmit path with regards to using destroyed mutex in mlx5en(4).

It may happen during link down that the running state may be observed
non-zero in the transmit routine, right before the running state is
cleared. This may end up using a destroyed mutex.

Make all channel mutexes and callouts persistant.

Preserve receive and send queue statistics during link toggle.

MFC after: 3 days
Sponsored by: Mellanox Technologies