Add support for --fsync and --fdatasync in io_uring_cmd ioengine to
enable FLUSH commands just like libaio or io_uring ioengines.

If --fsync or --fdatasync is given N, FLUSH command will be issued as
per N write commands.

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

`last_ddir` represents the data direction of the latest completed
command.  To avoid confusions, this patch renamed `last_ddir` to
`last_ddir_completed` to make it much more clear.

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

`last_was_sync` has represented that the last command had DDIR_SYNC.
This can be replaced with `ddir_sync(last_ddir_issued)` and it's much
more flexible to represent the last issued command's data direction.

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

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>

Even if ddir is determined in get_rw_ddir(), ddir might be updated in
set_rw_ddir().  if td represents trimwrite, it will be updated to either
DDIR_TRIM or DDIR_WRITE even ddir already represents for DDIR_SYNC.

To support DDIR_SYNC(fsync) for trimwrite, this patch checks ddir_sync()
in case of trimwrite not to update the pre-determined ddir.

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

This test script tests number of FLUSH commands triggered by --fsync=<N>
options to make FLUSH commands are followed by the WRITE commands from
the various --rw I/O workload.

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