netdevsim: change debugfs tree topology

With the model where dev is represented by devlink and ports are
represented by devlink ports, make debugfs file names independent
on netdev names. Change the topology to the one illustrated
by the following example:

$ ls /sys/kernel/debug/netdevsim/
netdevsim1
$ ls /sys/kernel/debug/netdevsim/netdevsim1/
bpf_bind_accept  bpf_bind_verifier_delay  bpf_bound_progs  ports
$ ls /sys/kernel/debug/netdevsim/netdevsim1/ports/
0  1
$ ls /sys/kernel/debug/netdevsim/netdevsim1/ports/0/
bpf_map_accept  bpf_offloaded_id  bpf_tc_accept  bpf_tc_non_bound_accept  bpf_xdpdrv_accept  bpf_xdpoffload_accept  dev  ipsec
$ ls /sys/kernel/debug/netdevsim/netdevsim1/ports/0/dev -l
lrwxrwxrwx 1 root root 0 Apr 13 15:58 /sys/kernel/debug/netdevsim/netdevsim1/ports/0/dev -> ../../../netdevsim1

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