tun: rename generic_xdp to skb_xdp

Rename "generic_xdp" to "skb_xdp" to avoid confusing it with the
generic XDP which will be done at netif_receive_skb().

Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>