selftests: xsk: Make frame_size configurable

Make the frame size configurable instead of it being hard coded to a
default. This is a property of the umem and will make it possible to
implement tests for different umem frame sizes in a later patch.

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/20210907071928.9750-7-magnus.karlsson@gmail.com