netdevsim: Add support for resilient nexthop groups

Allow resilient nexthop groups to be programmed and account their
occupancy according to their number of buckets. The nexthop group itself
as well as its buckets are marked with hardware flags (i.e.,
'RTNH_F_TRAP').

Replacement of a single nexthop bucket can fail using the following
debugfs knob:

 # cat /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_nexthop_bucket_replace
 N
 # echo 1 > /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_nexthop_bucket_replace
 # cat /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_nexthop_bucket_replace
 Y

Replacement of a resilient nexthop group can fail using the following
debugfs knob:

 # cat /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_res_nexthop_group_replace
 N
 # echo 1 > /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_res_nexthop_group_replace
 # cat /sys/kernel/debug/netdevsim/netdevsim10/fib/fail_res_nexthop_group_replace
 Y

This enables testing of various error paths.

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>