netdevsim: fib: Add debugfs knob to simulate route deletion failure

The previous patch ("netdevsim: fib: Fix reference count leak on route
deletion failure") fixed a reference count leak that happens on route
deletion failure.

Such failures can only be simulated by injecting slab allocation
failures, which cannot be surgically injected.

In order to be able to specifically test this scenario, add a debugfs
knob that allows user space to fail route deletion requests when
enabled.

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