zfs_sync: return error when pool suspends

If the pool is suspended, we'll just block in zil_commit(). If the
system is shutting down, blocking wouldn't help anyone. So, we should
keep this test for now, but at least return an error for anyone who is
actually interested.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17420