Assert if we're logging after final txg was set

This allowed to debug #16714, fixed in #16782. Without assertions
added here it is difficult to figure out what logs cause the problem,
since the assertion happens in sync thread context.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Pavel Snajdr <snajpa@snajpa.net>
Co-authored-by: Alexander Motin <mav@FreeBSD.org>
Closes #16795