devlink: extend pr_out_nested_handle() to print object

devlink: extend pr_out_nested_handle() to print object

For existing pr_out_nested_handle() user (line card), the output stays
the same. For the new users, introduce __pr_out_nested_handle()
to allow to print devlink instance as object allowing to carry
attributes in it (like netns).

Note that as __pr_out_handle_start() and pr_out_handle_end() are newly
used, the function is moved below the definitions.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>