Initialize sequence page under buffer content lock

Shared buffer access rules mandate that a pin as well as content lock
in exclusive mode is needed to update a shared buffer.

src/backend/commands/sequence.c

@@ -383,14 +383,14 @@ fill_seq_with_data(Relation rel, HeapTuple tuple)
 
 	page = BufferGetPage(buf);
 
+	LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
+
 	PageInit(page, BufferGetPageSize(buf), sizeof(sequence_magic));
 	sm = (sequence_magic *) PageGetSpecialPointer(page);
 	sm->magic = SEQ_MAGIC;
 
 	/* Now insert sequence tuple */
 
-	LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
-
 	/*
 	 * Since VACUUM does not process sequences, we have to force the tuple
 	 * to have xmin = FrozenTransactionId now.	Otherwise it would become