net: xdp: allow generic and driver XDP on one interface

Since commit a25717d2

 ("xdp: support simultaneous driver and
hw XDP attachment") users can load an XDP program for offload and
in native driver mode simultaneously.  Allow a similar mix of
offload and SKB mode/generic XDP.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>