devlink: Move devlink health get and set code to health file

Move devlink health get and set callbacks and related code from
leftover.c to health.c. No functional change in this patch.

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>