checkpatch: Warn about data_race() without comment

Warn about applications of data_race() without a comment, to encourage
documenting the reasoning behind why it was deemed safe.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>