locking/lockdep: Shrink struct lock_class_key

Shrink struct lock_class_key; we never store anything in subkeys[], we
only use the addresses.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/lockdep.h

@@ -58,8 +58,10 @@ struct lockdep_subclass_key {
 
 /* hash_entry is used to keep track of dynamically allocated keys. */
 struct lock_class_key {
-	struct hlist_node		hash_entry;
-	struct lockdep_subclass_key	subkeys[MAX_LOCKDEP_SUBCLASSES];
+	union {
+		struct hlist_node		hash_entry;
+		struct lockdep_subclass_key	subkeys[MAX_LOCKDEP_SUBCLASSES];
+	};
 };
 
 extern struct lock_class_key __lockdep_no_validate__;