bpf: add support for generic xdp

bpf: add support for generic xdp

Follow-up to commit c7272ca72009 ("bpf: add initial support for
attaching xdp progs") to also support generic XDP. This adds an
indicator for loaded generic XDP programs when programs are loaded
as shown in c7272ca72009, but the driver still lacks native XDP
support.

  # ip link
  [...]
  3: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 xdpgeneric qdisc [...]
      link/ether 0c:c4:7a:03:f9:25 brd ff:ff:ff:ff:ff:ff
  [...]

In case the driver does support native XDP, but the user wants
to load the program as generic XDP (e.g. for testing purposes),
then this can be done with the same semantics as in c7272ca72009,
but with 'xdpgeneric' instead of 'xdp' command for loading:

  # ip -force link set dev eno1 xdpgeneric obj xdp.o

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David S. Miller <davem@davemloft.net>