Improve l2arc reporting in arc_summary.

• Do not report L2ARC as FAULTED in presence of in-flight writes.
• Report read and write I/Os, bytes and errors.
• Remove few numbers not important to average user.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #12304
Closes #14946