L2ARC: Move different stats updates earlier

..., before we make the header or the log block visible to others.
It should fix assertion on allocated space going negative if the
header is freed once the lock is dropped, while the write is still
going.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16040
Closes #16743