devlink: Add health buffer support

Devlink health buffer is a mechanism to pass descriptors between drivers
and devlink. The API allows the driver to add objects, object pair,
value array (nested attributes), value and name.

Driver can use this API to fill the buffers in a format which can be
translated by the devlink to the netlink message.

In order to fulfill it, an internal buffer descriptor is defined. This
will hold the data and metadata per each attribute and by used to pass
actual commands to the netlink.

This mechanism will be later used in devlink health for dump and diagnose
data store by the drivers.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>