Add missing parenthesis in VERIFYF()

Without them the order of operations might get unexpected.

Reviewed-by: Adam Moss <c@yotes.com>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16826