net/failsafe: replace local device with shared data

net/failsafe: replace local device with shared data

In multiprocess context, the private structure is shared between
processes. The back reference from private to generic data was using
a pointer to a per process eth_dev. It's now changed to a reference of
the shared data.

Signed-off-by: Raslan Darawsheh <rasland@mellanox.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Gaetan Rivet <gaetan.rivet@6wind.com>