checkpatch: Error out if deprecated RCU API used

Single-argument kvfree_rcu() usage is being deprecated [1] [2]. However,
till all users are converted, we would like to introduce checkpatch
errors for new patches submitted.

This patch adds support for the same. Tested with a trial patch.

For now, we are only considering usages that don't have compound
nesting, for example ignore: kvfree_rcu( (rcu_head_obj), rcu_head_name).
This is sufficient as such usages are unlikely.

Once all users are converted and we remove the old API, we can also revert this
checkpatch patch then.

[1] https://lore.kernel.org/rcu/CAEXW_YRhHaVuq+5f+VgCZM=SF+9xO+QXaxe0yE7oA9iCXK-XPg@mail.gmail.com/
[2] https://lore.kernel.org/rcu/CAEXW_YSY=q2_uaE2qo4XSGjzs4+C102YMVJ7kWwuT5LGmJGGew@mail.gmail.com/



Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>