devlink: use min_t to calculate data_size

The calculation for the data_size in the devlink_nl_read_snapshot_fill
function uses an if statement that is better expressed using the min_t
macro.

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>