selftests/xsk: Avoid bpf_link probe for existing xsk

Currently bpf_link probe is done for each call of xsk_socket__create().
For cases where xsk context was previously created and current socket
creation uses it, has_bpf_link will be overwritten, where it has already
been initialized.

Optimize this by moving the query to the xsk_create_ctx() so that when
xsk_get_ctx() finds a ctx then no further bpf_link probes are needed.

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/20220629143458.934337-2-maciej.fijalkowski@intel.com