net/mlx4_en: Avoid scheduling restart task if it is already running

Add restarting state flag to avoid scheduling another restart task while
such task is already running. Change task name from watchdog_task to
restart_task to better fit the task role.

Fixes: 1e338db5 ("mlx4_en: Fix a race at restart task")
Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>