selftest: netdevsim: test devlink default params

selftest: netdevsim: test devlink default params

Test querying default values and resetting to default values for
netdevsim devlink params.

This should cover the basic paths of interest: driverinit and
non-driverinit cmodes, as well as bool and non-bool value
type. Default param values of type bool are encoded with u8 netlink
type as opposed to flag type, so that userspace can distinguish
"not-present" from false.

Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20251119025038.651131-7-daniel.zahka@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>