bufmgr: Fix signedness of mask variable in BufferSync()

BM_PERMANENT is defined as 1U<<31, which is a negative number when interpreted
as a signed integer. Unfortunately the mask variable in BufferSync() was
signed. This has been wrong for a long time, but failed to fail, due to
integer conversion rules.

However, in an upcoming patch the width of the state variable will be
increased, with the wrong signedness leading to never flushing permanent
buffers - luckily caught in a test.

It seems better to fix this separately, instead of doing so as part of a
large, otherwise mechanical, patch.

Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff

src/backend/storage/buffer/bufmgr.c

@@ -3321,7 +3321,7 @@ BufferSync(int flags)
 	Oid			last_tsid;
 	binaryheap *ts_heap;
 	int			i;
-	int			mask = BM_DIRTY;
+	uint32		mask = BM_DIRTY;
 	WritebackContext wb_context;
 
 	/*