lib/scanner: Recheck file size and modification time after hashing (ref #3440)

To catch the case where the file changed. Also make sure we never let a
size-vs-blocklist mismatch slip through.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3443

lib/model/rwfolder_test.go

@@ -223,7 +223,7 @@ func TestCopierFinder(t *testing.T) {
 	}
 
 	// Verify that the fetched blocks have actually been written to the temp file
-	blks, err := scanner.HashFile(tempFile, protocol.BlockSize, 0, nil)
+	blks, err := scanner.HashFile(tempFile, protocol.BlockSize, nil)
 	if err != nil {
 		t.Log(err)
 	}