Correct value of LW_SHARED_MASK.

The previous wrong value lead to wrong LOCK_DEBUG output, never showing
any shared lock holders.

Reported-By: Alexander Korotkov
Discussion: CAPpHfdsPmWqz9FB0AnxJrwp1=KLF0n=-iB+QvR0Q8GSmpFVdUQ@mail.gmail.com
Backpatch: 9.5, where the bug was introduced.

src/backend/storage/lmgr/lwlock.c

@@ -107,7 +107,7 @@ extern slock_t *ShmemLock;
 
 #define LW_LOCK_MASK				((uint32) ((1 << 25)-1))
 /* Must be greater than MAX_BACKENDS - which is 2^23-1, so we're fine. */
-#define LW_SHARED_MASK				((uint32)(1 << 23))
+#define LW_SHARED_MASK				((uint32) ((1 << 24)-1))
 
 /*
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