devlink: Split out health reporter create code

Move devlink health reporter create/destroy and related dev code to new
file health.c. This file shall include all callbacks and functionality
that are related to devlink health.

In addition, fix kdoc indentation and make reporter create/destroy kdoc
more clear. No functional change in this patch.

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