selftests/xsk: Introduce XDP prog load based on existing AF_XDP socket

Currently, xsk_setup_xdp_prog() uses anonymous xsk_socket struct which
means that during xsk_create_bpf_link() call, xsk->config.xdp_flags is
always 0. This in turn means that from xdpxceiver it is impossible to
use xdpgeneric attachment, so since commit 3b22523b

 ("selftests,
xsk: Fix bpf_res cleanup test") we were not testing SKB mode at all.

To fix this, introduce a function, called xsk_setup_xdp_prog_xsk(), that
will load XDP prog based on the existing xsk_socket, so that xsk
context's refcount is correctly bumped and flags from application side
are respected. Use this from xdpxceiver side so we get coverage of
generic and native XDP program attach points.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220629143458.934337-3-maciej.fijalkowski@intel.com