Use macros for calculating LWLock offset

Hi,

In the lwlock.c, InitializeLWLocks() calculate the LWLock offset by
itself (c319991bcad),
however, there are macros defined in lwlock.h, I think, we can use the
macros.

diff --git a/src/backend/storage/lmgr/lwlock.c
b/src/backend/storage/lmgr/lwlock.c
index 2fa90cc095..108e652179 100644
--- a/src/backend/storage/lmgr/lwlock.c
+++ b/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);

--
Best regards
Japin Li