MDEV-15059 - Misc small InnoDB scalability fixes

Moved mutex locking inside lock_rec_lock().
Moved monitor increment out of mutex.
Moved assertions that don't require protection out of mutex.
Removed duplicate assertions.
Moved duplicate debug injections into lock_rec_lock().
Let monitor updates use relaxed memory order.
Return directly without maintaining variables in lock_rec_lock_slow().
Moved lock_rec_lock_fast() body into lock_rec_lock(): saves at least one
trx_mutex_enter(), one switch() plus some code was moved out of mutex.

storage/innobase/include/lock0priv.h

@@ -745,7 +745,7 @@ class RecLock {
 #ifdef WITH_WSREP
 		,lock_t*	c_lock = NULL
 #endif /* WITH_WSREP */
-	);
+	) const;
 
 	/**
 	Check of the lock is on m_rec_id.
@@ -838,7 +838,7 @@ class RecLock {
 	@param[in,out] lock	Newly created record lock to add to the
 				rec hash and the transaction lock list
 	@param[in] add_to_hash	If the lock should be added to the hash table */
-	void lock_add(lock_t* lock, bool add_to_hash);
+	void lock_add(lock_t* lock, bool add_to_hash) const;
 
 	/**
 	Check and resolve any deadlocks

storage/innobase/include/srv0mon.h

@@ -608,8 +608,9 @@ Use MONITOR_INC if appropriate mutex protection exists.
 #define MONITOR_ATOMIC_INC_LOW(monitor, enabled)			\
 	if (enabled) {							\
 		ib_uint64_t	value;					\
-		value  = my_atomic_add64(				\
-			(int64*) &MONITOR_VALUE(monitor), 1) + 1;	\
+		value  = my_atomic_add64_explicit(			\
+			(int64*) &MONITOR_VALUE(monitor), 1,		\
+			MY_MEMORY_ORDER_RELAXED) + 1;			\
 		/* Note: This is not 100% accurate because of the	\
 		inherent race, we ignore it due to performance. */	\
 		if (value > (ib_uint64_t) MONITOR_MAX_VALUE(monitor)) {	\
@@ -624,8 +625,9 @@ Use MONITOR_DEC if appropriate mutex protection exists.
 #define MONITOR_ATOMIC_DEC_LOW(monitor, enabled)			\
 	if (enabled) {							\
 		ib_uint64_t	value;					\
-		value = my_atomic_add64(				\
-			(int64*) &MONITOR_VALUE(monitor), -1) - 1;	\
+		value = my_atomic_add64_explicit(			\
+			(int64*) &MONITOR_VALUE(monitor), -1,		\
+			MY_MEMORY_ORDER_RELAXED) - 1;			\
 		/* Note: This is not 100% accurate because of the	\
 		inherent race, we ignore it due to performance. */	\
 		if (value < (ib_uint64_t) MONITOR_MIN_VALUE(monitor)) {	\