selftests: xsk: Move rxqsize into xsk_socket_info

Move the global variable rxqsize to struct xsk_socket_info as it
describes the size of a ring in that struct. By default, it is set to
the size dictated by libbpf.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210907071928.9750-6-magnus.karlsson@gmail.com