netlink: specs: fix up truthy values

Clean up all truthy value warnings reported by yamllint in the
netlink specs:

    warning  truthy value should be one of [false, true]  (truthy)

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> # mptcp_pm.yaml
Link: https://patch.msgid.link/20250610125944.85265-5-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>