LinuxKPI: add DEFINE_LOCK_GUARD_0 for rcu

This adds guard support for non-real-types like rcu locking meaning
that we need to keep the lock state separately ourselves.  _T is still
special and needs to be updated.  Given it may not be used it needs
an __unused attribute (we are using the LinuxKPI __maybe_unused which
indeed is more expressive in this case).

Sponsored by:	The FreeBSD Foundation (initially)
MFC after:	3 days
Reviewed by:	dumbbell
Differential Revision:	https://reviews.freebsd.org/D52076

Author: Bjoern A. Zeeb

sys/compat/linuxkpi/common/include/linux/cleanup.h

@@ -55,4 +55,39 @@
 
 #define	__free(_n)		__cleanup(__free_##_n)
 
+/*
+ * Given this is a _0 version it should likely be broken up into parts.
+ * But we have no idead what a _1, _2, ... version would do different
+ * until we see a call.
+ * This is used for a not-real-type (rcu).   We use a bool to "simulate"
+ * the lock held.  Also _T still special, may not always be used, so tag
+ * with __unused (or better the LinuxKPI __maybe_unused).
+ */
+#define	DEFINE_LOCK_GUARD_0(_n, _lock, _unlock, ...)			\
+									\
+    typedef struct {							\
+	bool lock;							\
+	__VA_ARGS__;							\
+    } guard_ ## _n ## _t;	    					\
+									\
+    static inline void							\
+    guard_ ## _n ## _destroy(guard_ ## _n ## _t *_T)			\
+    {									\
+	if (_T->lock) {							\
+	    _unlock;							\
+	}								\
+    }									\
+									\
+    static inline guard_ ## _n ## _t					\
+    guard_ ## _n ## _create(void)					\
+    {									\
+	guard_ ## _n ## _t _tmp;					\
+	guard_ ## _n ## _t *_T __maybe_unused;				\
+									\
+	_tmp.lock = true;						\
+	_T = &_tmp;							\
+	_lock;								\
+	return (_tmp);							\
+    }
+
 #endif	/* _LINUXKPI_LINUX_CLEANUP_H */

sys/compat/linuxkpi/common/include/linux/rcupdate.h

@@ -1,7 +1,7 @@
 /*-
  * Copyright (c) 2016-2017 Mellanox Technologies, Ltd.
  * All rights reserved.
- * Copyright (c) 2024 The FreeBSD Foundation
+ * Copyright (c) 2024-2025 The FreeBSD Foundation
  *
  * Portions of this software were developed by Björn Zeeb
  * under sponsorship from the FreeBSD Foundation.
@@ -35,6 +35,7 @@
 #include <linux/compiler.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
+#include <linux/cleanup.h>
 
 #include <machine/atomic.h>
 
@@ -162,4 +163,6 @@ void linux_synchronize_rcu(unsigned type);
 #define	init_rcu_head_on_stack(...)
 #define	destroy_rcu_head_on_stack(...)
 
+DEFINE_LOCK_GUARD_0(rcu, rcu_read_lock(), rcu_read_unlock())
+
 #endif					/* _LINUXKPI_LINUX_RCUPDATE_H_ */