netdevsim: create devlink instance per netdevsim instance

Currently there is one devlink instance created per network namespace.
That is quite odd considering the fact that devlink instance should
represent an ASIC. The following patches are going to move the devlink
instance even more down to a bus device, but until then, have one
devlink instance per netdevsim instance. Struct nsim_devlink is
introduced to hold fib setting.

The changes in the fib code are only related to holding the
configuration per devlink instance instead of network namespace.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>