devlink: report extended error message in region_read_dumpit()

Report extended error details in the devlink_nl_cmd_region_read_dumpit()
function, by using the extack structure from the netlink_callback.

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