netdevsim: Allow reporting activity on nexthop buckets

A key component of the resilient hashing algorithm is the hash buckets'
activity. If a bucket is active, it will not be populated with a new
nexthop in order not to break existing flows. Therefore, in order to
easily and thoroughly test the algorithm, we need to be in full control
over the reported activity.

Add a debugfs interface that allows user space to have netdevsim report
a nexthop bucket within a resilient nexthop group as active. For
example:

 # echo 10 23 > /sys/kernel/debug/netdevsim/netdevsim10/fib/nexthop_bucket_activity

Will mark bucket 23 in nexthop group 10 as active.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>