The flag attr presence in Netlink message indicates value "true",
if it is missing in the message it means "false".

Allow user to specify attrname with value "true"/"false"
in json for flag attrs, treat "false" value properly.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240222134351.224704-2-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>