Use macros instead of hardcoded offsets for LWLock initialization

This makes the code slightly easier to follow, as the initialization
relies on an offset that overlapped with an equivalent set of macros
defined, which are used in other places already.

Author: Japin Li
Discussion: https://postgr.es/m/MEYP282MB1669FB410006758402F2C3A2B6E00@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

src/backend/storage/lmgr/lwlock.c

@@ -525,18 +525,17 @@ InitializeLWLocks(void)
 		LWLockInitialize(&lock->lock, id);
 
 	/* Initialize buffer mapping LWLocks in main array */
-	lock = MainLWLockArray + NUM_INDIVIDUAL_LWLOCKS;
+	lock = MainLWLockArray + BUFFER_MAPPING_LWLOCK_OFFSET;
 	for (id = 0; id < NUM_BUFFER_PARTITIONS; id++, lock++)
 		LWLockInitialize(&lock->lock, LWTRANCHE_BUFFER_MAPPING);
 
 	/* Initialize lmgrs' LWLocks in main array */
-	lock = MainLWLockArray + NUM_INDIVIDUAL_LWLOCKS + NUM_BUFFER_PARTITIONS;
+	lock = MainLWLockArray + LOCK_MANAGER_LWLOCK_OFFSET;
 	for (id = 0; id < NUM_LOCK_PARTITIONS; id++, lock++)
 		LWLockInitialize(&lock->lock, LWTRANCHE_LOCK_MANAGER);
 
 	/* Initialize predicate lmgrs' LWLocks in main array */
-	lock = MainLWLockArray + NUM_INDIVIDUAL_LWLOCKS +
-		NUM_BUFFER_PARTITIONS + NUM_LOCK_PARTITIONS;
+	lock = MainLWLockArray + PREDICATELOCK_MANAGER_LWLOCK_OFFSET;
 	for (id = 0; id < NUM_PREDICATELOCK_PARTITIONS; id++, lock++)
 		LWLockInitialize(&lock->lock, LWTRANCHE_PREDICATE_LOCK_MANAGER);