netdevsim: fib: Add debugfs to debug route offload failure

Add "fail_route_offload" flag to disallow offloading routes.
It is needed to test "offload failed" notifications.

Create the flag as part of nsim_fib_create() under fib directory and set
it to false by default.

When FIB_EVENT_ENTRY_{REPLACE, APPEND} are triggered and
"fail_route_offload" value is true, set the appropriate hardware flag to
make the kernel emit RTM_NEWROUTE notification with RTM_F_OFFLOAD_FAILED
flag.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>