net, neigh: Fix NTF_EXT_LEARNED in combination with NTF_USE

The NTF_EXT_LEARNED neigh flag is usually propagated back to user space
upon dump of the neighbor table. However, when used in combination with
NTF_USE flag this is not the case despite exempting the entry from the
garbage collector. This results in inconsistent state since entries are
typically marked in neigh->flags with NTF_EXT_LEARNED, but here they are
not. Fix it by propagating the creation flag to ___neigh_create().

Before fix:

  # ./ip/ip n replace 192.168.178.30 dev enp5s0 use extern_learn
  # ./ip/ip n
  192.168.178.30 dev enp5s0 lladdr f4:8c:50:5e:71:9a REACHABLE
  [...]

After fix:

  # ./ip/ip n replace 192.168.178.30 dev enp5s0 use extern_learn
  # ./ip/ip n
  192.168.178.30 dev enp5s0 lladdr f4:8c:50:5e:71:9a extern_learn REACHABLE
  [...]

Fixes: 9ce33e46 ("neighbour: support for NTF_EXT_LEARNED flag")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>