devlink: hold the instance lock during eswitch_mode callbacks

Make the devlink core hold the instance lock during eswitch_mode
callbacks. Cheat in case of mlx5 (see the cover letter).

Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>