devlink: Move devlink fmsg and health diagnose to health file

Devlink fmsg (formatted message) is used by devlink health diagnose,
dump and drivers which support these devlink health callbacks.
Therefore, move devlink fmsg helpers and related code to file health.c.
Move devlink health diagnose to file 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>