Add zfetch stats in arcstats

arc_summary also reports zfetch stats but it's inconvenient to monitor
contiguously incrementing numbers. Adding them in arcstats allows us to
observe streams more conveniently.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #16094