reftable/stack: don't call fsync(3p) unless provided

Users of the reftable library are expected to provide their own function
callback in cases they want to sync(3p) data to disk via the reftable
write options. But if no such function was provided we end up calling
fsync(3p) directly, which may not even be available on some systems.

Drop the call to fsync(3p) and rely on the callback function
exclusively.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

reftable/stack.c

@@ -33,7 +33,7 @@ static int stack_fsync(const struct reftable_write_options *opts, int fd)
 {
 	if (opts->fsync)
 		return opts->fsync(fd);
-	return fsync(fd);
+	return 0;
 }
 
 static ssize_t reftable_write_data(int fd, const void *data, size_t size)