net/mlx4: Lock mlx4 devlink reload callback

Change devlink instance locks in mlx4 driver to have devlink reload
callback locked, while keeping all driver paths which leads to devl_ API
functions called by the mlx4 driver locked.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>