devlink: health: Fix nla_nest_end in error flow

devlink_nl_health_reporter_fill() error flow calls nla_nest_end(). Fix
it to call nla_nest_cancel() instead.

Note the bug is harmless as genlmsg_cancel() cancel the entire message,
so no fixes tag added.

Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>