devlink: Add devlink formatted message (fmsg) API

Devlink fmsg is a mechanism to pass descriptors between drivers and
devlink, in json-like format. The API allows the driver to add nested
attributes such as object, object pair and value array, in addition to
attributes such as name and value.

Driver can use this API to fill the fmsg context in a format which will be
translated by the devlink to the netlink message later.
There is no memory allocation in advance (other than the initial list
head), and it dynamically allocates messages descriptors and add them to
the list on the fly.

When it needs to send the data using SKBs to the netlink layer, it
fragments the data between different SKBs. In order to do this
fragmentation, it uses virtual nests attributes, to avoid actual
nesting use which cannot be divided between different SKBs.

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