io_u: Ensure fsync only after write(s)

When using `--rw=write --fsync=N`, the FLUSH command is correctly
issued after N WRITE commands. However, if READ commands are mixed
in with --rw, fsync occurs after READ commands as well. This patch
ensures that fsync is only triggered after the specified number of
WRITE commands, regardless of READ commands.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>

io_u.c

@@ -755,7 +755,7 @@ static enum fio_ddir get_rw_ddir(struct thread_data *td)
  * See if it's time to fsync/fdatasync/sync_file_range first,
  * and if not then move on to check regular I/Os.
  */
- if (should_fsync(td)) {
+ if (should_fsync(td) && td->last_ddir_issued == DDIR_WRITE) {
  if (td->o.fsync_blocks && td->io_issues[DDIR_WRITE] &&
  !(td->io_issues[DDIR_WRITE] % td->o.fsync_blocks))
  return DDIR_SYNC;