selftests/xsk: add metadata copy test for multi-buff

Enable the already existing metadata copy test to also run in
multi-buffer mode with 9K packets.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230719132421.584801-23-maciej.fijalkowski@intel.com


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