Optimize the is_l2cacheable functions

by placing the most common use case (no special vdevs) first and avoid
allocating new variables.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14494
Closes #14563