Fix zenith_test_evict mode and clear_buffer_cache() function

Using InvalidateBuffer is wrong, because if the page is concurrently
dirtied, it will throw away the dirty page without calling
smgwrite(). In Neon, that means that the last-written LSN update for
the page is missed.

In v16, use the new InvalidateVictimBuffer() function that does what
we need. In v15 and v14, backport the InvalidateVictimBuffer()
function.

Fixes issue neondatabase/neon#7802

src/backend/storage/buffer/bufmgr.c

@@ -2473,24 +2473,26 @@ UnpinBuffer(BufferDesc *buf)
 		if (zenith_test_evict && !InRecovery)
 		{
 			buf_state = LockBufHdr(buf);
-			if (BUF_STATE_GET_REFCOUNT(buf_state) == 0)
+			if ((buf_state & BM_VALID) && BUF_STATE_GET_REFCOUNT(buf_state) == 0)
 			{
+				ReservePrivateRefCountEntry();
+				PinBuffer_Locked(buf);
 				if (buf_state & BM_DIRTY)
 				{
-					ReservePrivateRefCountEntry();
-					PinBuffer_Locked(buf);
 					if (LWLockConditionalAcquire(BufferDescriptorGetContentLock(buf),
 												 LW_SHARED))
 					{
 						FlushOneBuffer(b);
 						LWLockRelease(BufferDescriptorGetContentLock(buf));
 					}
-					UnpinBuffer(buf);
-				}
-				else
-				{
-					InvalidateBuffer(buf);
 				}
+
+				/*
+				 * Try to invalidate the page. This can fail if the page is
+				 * concurrently modified; we'll let it be in that case
+				 */
+				(void) InvalidateVictimBuffer(buf);
+				UnpinBuffer(buf);
 			}
 			else
 				UnlockBufHdr(buf, buf_state);