Refactor Log Size Limit

Original Log Size Limit implementation blocked all writes in case of
limit reached until the TXG is committed and the log is freed. It
caused huge delays and following speed spikes in application writes.

This implementation instead smoothly throttles writes, using exactly
the same mechanism as used for dirty data.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: jxdking <lostking2008@hotmail.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Issue #12284
Closes #13476