selftests: xsk: Move num_frames and frame_headroom to xsk_umem_info

Move the global variables num_frames and frame_headroom to struct
xsk_umem_info. They describe properties of the umem so no reason for
them to be global.

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/20210907071928.9750-5-magnus.karlsson@gmail.com