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>

scripts/checkpatch.pl

@@ -6388,6 +6388,15 @@ sub process {
 			}
 		}
 
+# check for soon-to-be-deprecated single-argument k[v]free_rcu() API
+		if ($line =~ /\bk[v]?free_rcu\s*\([^(]+\)/) {
+			if ($line =~ /\bk[v]?free_rcu\s*\([^,]+\)/) {
+				ERROR("DEPRECATED_API",
+				      "Single-argument k[v]free_rcu() API is deprecated, please pass rcu_head object or call k[v]free_rcu_mightsleep()." . $herecurr);
+			}
+		}
+
+
 # check for unnecessary "Out of Memory" messages
 		if ($line =~ /^\+.*\b$logFunctions\s*\(/ &&
 		    $prevline =~ /^[ \+]\s*if\s*\(\s*(\!\s*|NULL\s*==\s*)?($Lval)(\s*==\s*NULL\s*)?\s*\)/ &&