netkit: Reject IFLA_NETKIT_PEER_INFO in netkit_change_link

The IFLA_NETKIT_PEER_INFO attribute can only be used during device
creation, but not via changelink callback. Hence reject it there.

Fixes: 35dfaad7

 ("netkit, bpf: Add bpf programmable net device")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/e86a277a1e8d3b19890312779e42f790b0605ea4.1701115314.git.daniel@iogearbox.net


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>