net/mlx5: Fix deadlock between devlink lock and esw->wq

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

net/mlx5: Fix deadlock between devlink lock and esw->wq

esw->work_queue executes esw_functions_changed_event_handler ->

esw_vfs_changed_event_handler and acquires the devlink lock.

.eswitch_mode_set (acquires devlink lock in devlink_nl_pre_doit) ->

mlx5_devlink_eswitch_mode_set -> mlx5_eswitch_disable_locked ->

mlx5_eswitch_event_handler_unregister -> flush_workqueue deadlocks

when esw_vfs_changed_event_handler executes.

Fix that by no longer flushing the work to avoid the deadlock, and using

a generation counter to keep track of work relevance. This avoids an old

handler manipulating an esw that has undergone one or more mode changes:

- the counter is incremented in mlx5_eswitch_event_handler_unregister.

- the counter is read and passed to the ephemeral mlx5_host_work struct.

- the work handler takes the devlink lock and bails out if the current

generation is different than the one it was scheduled to operate on.

- mlx5_eswitch_cleanup does the final draining before destroying the wq.

No longer flushing the workqueue has the side effect of maybe no longer

cancelling pending vport_change_handler work items, but that's ok since

those are disabled elsewhere:

- mlx5_eswitch_disable_locked disables the vport eq notifier.

- mlx5_esw_vport_disable disarms the HW EQ notification and marks

vport->enabled under state_lock to false to prevent pending vport

handler from doing anything.

- mlx5_eswitch_cleanup destroys the workqueue and makes sure all events

are disabled/finished.