devlink: factor out building a snapshot notification

We'll need to send snapshot info back on the socket
which requested a snapshot to be created. Factor out
constructing a snapshot description from the broadcast
notification code.

v3: new patch

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>