selftests: xsk: Simplify packet validation in xsk tests

Simplify packet validation in the xsk selftests by performing it at
once for every packet. The current code performed this per batch and
did this on copied packet data. Make it simpler and faster by
validating it at once and on the umem packet data thus skipping the
copy and the memory allocation for the temprary buffer.

The optional packet dump feature is also simplified in the same
manner. Memory allocation and copying is removed and the dump is
performed directly on the umem data.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-10-magnus.karlsson@gmail.com