zil: add stats for commit failure/fallback (#16315)

There's no good way to tell when a ZIL commit fails and falls back to a
transaction sync, other than perhaps a throughput drop. This adds
counters so we can see when it happens and why.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>