selftests/xsk: add unaligned mode test for multi-buffer

Add a test for multi-buffer AF_XDP when using unaligned mode. The test
sends 4096 9K-buffers.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230719132421.584801-21-maciej.fijalkowski@intel.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>