Help compiller optimize out abd_verify()

While abd_verify() does nothing when built without debug, compiler
can't optimize it out by itself due to calls to external list_*()
and abd_verify_scatter(). This commit makes it explicit.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Adam Moss <c@yotes.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12280