selftests: xsk: Generate packet directly in umem

Generate the packet directly in the umem instead of in a temporary
buffer that is copied out. Simplifies the code and improves
performance.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-14-magnus.karlsson@gmail.com