selftests: xsk: Remove the num_tx_packets option

Remove the number of tx packet option as this should be decided by the
test itself. Also change the number of packets to be sent to 4096
speeding up the execution.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-3-magnus.karlsson@gmail.com