zio: rename io_reexecute as io_post; use it for the direct IO checksum error flag

We're not supposed to modify someone else's io_flags, so we need another
way to propagate DIO_CHKSUM_ERR.

If we squint, we can see that io_reexecute is really just recording
exceptional events that a parent (or its parents) will need to do
something about. It just happens that the only things we've had
historically are two forms of reexecution: now or later (suspend).

So, rename it to io_post, as in, post-IO info/events/actions. And now we
have a few spare bits for other conditions.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17507