devlink: uniformly take the devlink instance lock in the dump loop

Move the lock taking out of devlink_nl_cmd_region_get_devlink_dumpit().
This way all dumps will take the instance lock in the main iteration
loop directly, making refactoring and reading the code easier.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>