zil: Add some more statistics.

In addition to a number of actual log bytes written, account also a
total written bytes including padding and total allocated bytes (bytes
<= write <= alloc). It should allow to monitor zil traffic and space
efficiency.

Add dtrace probe for zil block size selection.

Make zilstat report more information and fit it into less width.

Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14863