af_xdp: fix xdp socket create fail

af_xdp: fix xdp socket create fail

In libbpf code, xsk_socket__create will call xsk_link_lookup to get the
xdp_sock bpf prog. But xsk_link_lookup can't get any bpf prog. This will
cause Libbpf not to insert the fd into xsks_map and return ERROR.

The solution to this problem is to insert fd into xsks_map ourselves
instead of libbpf.

Type: fix
Change-Id: Ic5d279c6ddc02d67371262d6106a5b53b70e7913
Signed-off-by: Chen Yahui <goodluckwillcomesoon@gmail.com>